Ive been farming mp3 with nv5 (Not counting the 5th group as the 5th stack) Today its happened 3 times, 2 of which where in the same game. Ill kill the champion group (all of them) and not a single rare will drop. Its also happened in the past randomly probably 20-30 times to me. Spoke with hardcore chat and some people have the same problem others have played 300+ hours and never had it happen