Dark seer did not spawn.

I just got the "Final pack spawns the nameless seer on death" mod, in a waste pool map, on tier 14.
I full cleared the map, 0 monsters remaining. I also ran several laps through the map after i had cleared it to make sure i didn't miss anything.

The modifier spawned on those toxic sea witches, not sure what they are called.
Was playing on Frankfurt server. Not sure if there's any other information i can share that would help pinpoint what the issue was here?

same case with mine. waste pool map and nameless seer on the green monster.