Edgeyuna1200 did you ever end up contacting Blizzard? I had the same thing happen to me today. Of course I get a 12 hours ban, which completely destroys any hope of me being able to play at all for the rest of the day. And what did I get banned for? I have absolutely no idea. I wasn't muling or even joining games frequently. As you said, I am careful when joining/leaving games. In fact, I spend a minimum of 8 minutes in a game before I leave it. So after about 6 or 7 games of lengths ranging between 8 to 20 minutes, there's my 12 hour ban.
This is getting to be a joke. These realm restrictions don't do a damn thing to stop botting. Let's face it, bots are programmed to be in a game for a set amount of time and leave. The fact is these realm downs (which ARE in fact ALL 12 hours like you said) only hurt the legitimate players and not the actual bots. It's a real shame but it seems like that was Blizzards final stand at stopping botting. Throwing in an extremely ineffective measure that doesn't even effect botting. It's a shame, but it just goes to show you how little they actually care about this game. And I understand the game is over 10 years old, but that's no excuse for putting in the half day realm restrictions. If they want to use realm downs there is no reason for them to be any longer than 20 minutes to an hour.