There isn't a bug with iOS6 auto brightness.
It's just more intelligent. You have to train it in a way.
I go into a pitch black room with auto brightness on, and then give it a sec for the sensors to pickup on the darkness, and I turn the brightness lower with auto brightness toggled on, then i walk into a room with ambient light, and turn the brightness higher...
go back into the room and back into the light, and itll adjust to the levels you set per darkness and light, and will adjust within those parameters.
You have to initially setup the auto brightness to the levels in darkness and light, and itll auto adjust to those levels from now on.