This happens in TranslateWiki. Special:Version there says "Version 2.0-alpha r64515".
Message only becomes read after i click the "Mark as read" button. I would expect that it would be automatically marked as read after i view it, but it stays unread even after i reply to it.
See also: http://translatewiki.net/wiki/Thread:Support/Messages_stay_unread
Version: unspecified
Severity: enhancement