Jenkins.reload() doesn't re-load global configs for extensions from the disk. The CLI reload-configuration is affected as well. The CLI reload-configuration help message is Discard all the loaded data in memory and reload everything from file system. Useful when you modified config files directly on disk. which isn't true.

That means e.g. manual modification on the disk for $JENKINS_HOME/hudson.tasks.Mailer.xml isn't re-loaded, only restart solves that issue.

It isn't covered by the JTH too, there is only reloadDescriptorConfig() test but not reloadDescriptorConfigUsingGlobalReload() - see reloadUserConfig() and reloadUserConfigUsingGlobalReload(). That could be the reason why it was hidden so long time.