Did you ever find a solution for this? I'm having the same issue.
I'm not sure when it started as I have a habit of putting the phone to sleep manually before I holster it.
If I lock the phone, the screen will dim, and at intervals, get bright again on its own. If I leave it at the TF Home Screen, it will stay bright and never dim.
I just can't bring myself to hard reset this phone right now . . lol. Maybe when I have more time. For now, I'll keep manually putting the phone to sleep .. . . sort of a muscle memory thing anyway. I wouldn't mind fixing it though, either.