I upgraded according to the guide, and suddenly all my multibranch jobs are gone from Jenkins. I look at the logs and see this for each of my job types:

Failed Loading item <job name>
java.lang.NullPointerException
	at jenkins.branch.MultiBranchProject.onLoad(MultiBranchProject.java:171)
	at hudson.model.Items.load(Items.java:372)
	at jenkins.model.Jenkins$17.run(Jenkins.java:3061)
	at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
	at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
	at jenkins.model.Jenkins$7.runTask(Jenkins.java:1066)
	at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
	at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)

I can recreate the jobs with the same names, upon which Jenkins reindexes from GitHub, "finds" the jobs for still-open branches and PRs, including their whole histories, and everything else works as it should. Old jobs still don't re-appear.

But then when I restart Jenkins they're all gone again, and I have to do the whole dance again.

As you can imagine, this is incredibly frustrating and a terrible time sink. What can I do to fix this, short of creating brand new jobs with lost history? I've tried to downgrade but the dependencies are too entangled. I have a backup of my plugin directory and plugin configs, but I'd really rather avoid regressive steps like that.