Amount of pregenerated monsters
According to wands of monster detection, amount of monsters in freshly generated dungeon levels seems to be 40-50 on higher danger levels. I do not have exact numbers on p20 behavior, but it was around maybe 10, with 20 being the limit, and 40 is enough to completely pacify even Big Room.
Is this such drastic change intented, and everything is fine? No calling "PopulateLevel" two times, or anything? It may be a little too much monsters. While 20-30 in my opinion is perfectly fine on high DL - 40-50 is over the top.