Sometimes after killing a nemesis group, another nemesis of a different family spawns.
I encountered around 39 nemesis spawns with over 400 battles in one realm.
I just noticed that when this happens, you don’t get any reward for the nemesis you just killed.
Edit: Seems like sometimes you do and sometimes you don’t.
Also, I just killed a 6-waspid pack and a nemesis dragon appeared, so it’s not limited to other nemeses.