Yup, sadly, we thought we had this one fixed (not reproducable in dev), but it's not. To be clear, threads moving to Site Feedback is actually a different bug than the one I mentioned at the beginning of this thread. That one made threads leave all forums. This one just makes them to go Site Feedback. There's some weirdness with how we get the data for each post on edit. We're going to overhaul it and just remove the possibility of the problem (since we can't seem to figure it out). :-\