Entered the QA and had zero spawns. Left and reentered and only the cursed marching undead spawned, but nothing else. Still was able to complete the quest and received end xp.
This happened to me once awhile back. Don't remember if I reported it or not. It seems like a weird bug that just happens from time to time.
I did this quest once today, got my butt kicked by the zombies, failed the quest, and closed it.
Later on, I came back with another PC and got no spawns.
Tried it again after a reset and it worked fine.
Hopefully fixed.