We've repeatedly lost flags in https://bugzilla.redhat.com/show_bug.cgi?id=647797 when adding comments or changing bug state. See the history of the bug.
Can be related to bug 838095.
I have had to reset acks on bugs when devels change their status to MODIFIED, and acks are accidentally reset to ?. Devels say no Bugzilla collision notification is given. Note this is not a new problem, but a persistent old one.
*** This bug has been marked as a duplicate of bug 838095 ***