Set logging level as "Diagnostic" closes log appender

Each time I try to raise the level of logging to "Diagnostic", the appender is closing. Result is no more log until restart of confluence :-/

To reproduce:
Admin console -> Logging & Profiling -> Click on "Diagnostic"

The action throws a last line in "attlassian-confluence,log":
2015-03-23 14:15:27,160 DEBUG [http-bio-8090-exec-6] [atlassian.confluence.logging.ConfluenceHomeLogAppender] shutdown closing appender

No more entry in the log until restart of Confluence, which reset the loging level to "Production".

The issue occurs since several releases (I've listed the 3 ones we used in production).