The problem maybe related to JENKINS-23952 and JENKINS-64088.  However the variant is slightly different

Prior to 2.273, we had numerous jobs with multiple post build tasks (working), and those continue work as long as they're not edited (in 2.273).   However if you edit a job (to add a task to the Post Build) that's when things break - you can add the task, script information, etc.; you can hit apply or save, and it appears to save.  If you then exit the job and come back in, only the first two tasks are preserved, all remaining tasks are lost

The only thing found in the system log is this message (but it's unclear if it's related)
Jan 11, 2021 3:18:09 PM WARNING org.kohsuke.stapler.framework.adjunct.AdjunctsInPage findNeeded
No such adjunct found: org.kohsuke.stapler.framework.adjunct.NoSuchAdjunctException: Neither .css, .js, .html, nor .jelly were found at org.kohsuke.stapler.framework.adjunct.Adjunct.<init>(Adjunct.java:125) at org.kohsuke.stapler.framework.adjunct.AdjunctManager.get(AdjunctManager.java:148) at org.kohsuke.stapler.framework.adjunct.AdjunctsInPage.findNeeded(AdjunctsInPage.java:161) at org.kohsuke.stapler.framework.adjunct.AdjunctsInPage.generate(AdjunctsInPage.java:113) at org.kohsuke.stapler.jelly.AdjunctTag.doTag(AdjunctTag.java:83) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.WhenTag.doTag(WhenTag.java:46) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ChooseTag.doTag(ChooseTag.java:38) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95) at org.kohsuke.stapler.jelly.IncludeTag.doTag(IncludeTag.java:147) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.OtherwiseTag.doTag(OtherwiseTag.java:41) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ChooseTag.doTag(ChooseTag.java:38) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.OtherwiseTag.doTag(OtherwiseTag.java:41) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ChooseTag.doTag(ChooseTag.java:38) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$1.run(CoreTagLibrary.java:98) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:100) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95) at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:64) at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:54) at org.kohsuke.stapler.jelly.ScriptInvoker.execute(ScriptInvoker.java:56) at org.kohsuke.stapler.jelly.ScriptInvoker.execute(ScriptInvoker.java:43) at org.kohsuke.stapler.ScriptRequestDispatcher.forward(ScriptRequestDispatcher.java:97) at org.jenkinsci.plugins.configfiles.ConfigFilesManagement.doAddConfig(ConfigFilesManagement.java:236) at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627) at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:396) at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:408) at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:212) at org.kohsuke.stapler.SelectionInterceptedFunction$Adapter.invoke(SelectionInterceptedFunction.java:36) at org.kohsuke.stapler.verb.HttpVerbInterceptor.invoke(HttpVerbInterceptor.java:48) at org.kohsuke.stapler.SelectionInterceptedFunction.bindAndInvoke(SelectionInterceptedFunction.java:26) at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:145) at org.kohsuke.stapler.MetaClass$11.doDispatch(MetaClass.java:536) at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:766) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:898) at org.kohsuke.stapler.MetaClass$9.dispatch(MetaClass.java:457) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:766) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:898) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:694) at org.kohsuke.stapler.Stapler.service(Stapler.java:240) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:791) at org.eclipse.jetty.servlet.ServletHandler$ChainEnd.doFilter(ServletHandler.java:1626) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:154) at jenkins.telemetry.impl.UserLanguages$AcceptLanguageFilter.doFilter(UserLanguages.java:129) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151) at jenkins.security.ResourceDomainFilter.doFilter(ResourceDomainFilter.java:76) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151) at jenkins.metrics.impl.MetricsFilter.doFilter(MetricsFilter.java:125) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151) at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:239) at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:215) at net.bull.javamelody.PluginMonitoringFilter.doFilter(PluginMonitoringFilter.java:88) at org.jvnet.hudson.plugins.monitoring.HudsonMonitoringFilter.doFilter(HudsonMonitoringFilter.java:114) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151) at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:157) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:153) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:92) at jenkins.security.AcegiSecurityExceptionFilter.doFilter(AcegiSecurityExceptionFilter.java:52) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:119) at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:113) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:105) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:101) at org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:92) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:218) at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:212) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:110) at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:80) at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:62) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97) at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:109) at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:168) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:51) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at jenkins.security.SuspiciousRequestFilter.doFilter(SuspiciousRequestFilter.java:36) at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601) at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:548) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143) at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:578) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127) at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:235) at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1624) at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233) at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1435) at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188) at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:501) at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1594) at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186) at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1350) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127) at org.eclipse.jetty.server.Server.handle(Server.java:516) at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:388) at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:633) at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:380) at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:273) at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311) at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:105) at org.eclipse.jetty.io.ChannelEndPoint$1.run(ChannelEndPoint.java:104) at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:336) at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:313) at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:171) at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:129) at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:375) at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:773) at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:905) at java.lang.Thread.run(Thread.java:748)