Nameless Seer not spawning after 0 monsters left

I ran a waste pool with 102% combined pack size increase and "back to basics" keystone. I had nameless seer to spawn on death of a "very low" pack, unknown type.

after 0 mobs left I spent 15 minutes looking at every out of map area and found nothing. there isn't even a chest left to click on. I have 3 possible ideas.

1: the very low pack never spawned due to small map
2: spawned outside of map then disappeared as it wasn't in a map
3: unlikely,but something with an item filter?

Thanks, good luck