When using the "Mark Node as temporarily offline" button, we are seeing them come back online after restarting Jenkins for maintenance. This seems to have changed somewhere in the last half year.

When looking at the agent "config.xml" after the restart, it still contains the same 

<temporaryOfflineCause class="hudson.slaves.OfflineCause$UserCause">
...
</temporaryOfflineCause>

block as before the restart, but the UI doesn't show it. After making an unrelated change to the description of the agent configuration via the UI, the block is removed from the config.xml.