This is a strange bug we've been trying to track down for a long time. We can't replicate it in dev. 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.