It reminds me of: http://www.phpbb.com/bugs/phpbb3/25235
the problem is, mnessage subject is not shortend by truncate_string() as post_subject and topic_title are.
Fix should be something like:
The same problem occures while saving drafts of PMs and Posts.
Also the problem wasn't fixed completely, as it still occures, while replying to a topic.
this is caused by
the fix for this behaviour is:
the same problem may occure with $append, just didn't find a place where it is used
While i was asking some points about this in the #phpbb-qa fawek confirmed this bug in the ACP and UCP