Not sure if this is being talked about yet but I've noticed it happening a few times. I'll start or join a expert room, start playing, get hit by a tank and not get downed and notice that the difficultly has been switched to Advanced. It happens each time you load the level, screwing with my "What are you trying to prove" achievement. You have to switch it back to expert every time a level loads. Pretty annoying. Hasn't happened every time I'm playing but it happens frequently. I emailed Gabe about it.