Floor may effect it tbh.
I was doing TreeParh quest and took me 2 hours to notice that the ones I killed inside the cave wasn't working lol.
After all we can't see their numbers. The target may be GreyEvilApe1 that spawns on 3F and GreyEvilApe2 spawns on 4F.
If this is the case, perhaps they should change the mobs or be more specific in the quest information