This is a strange bug we've been trying to track down for a long time.  We can't replicate it in dev.  Sad  Still looking into it.  It will only happen when you edit a root post that isn't in the Feedback forum.  We know what the problem is, but can't replicate it in dev (and it's not a data issue).  We'll probably end up just cutting out something it's doing with tags to "fix" it.  Good times.  Sorry about the weirdness.