Act 6 Fallen from Grace quest bug (Necromancer) - one cause

I was experiencing the bug again where you can't clear the zone for the quest only this time it would tell me I had 13-15 monsters remaining each time I would clear the map, and wouldn't progress the quest. I tried this while creating new instances four or five times and the same bug would repeat. On a hunch, given the number of monsters it said I had remaining, I despawned my spectre -- Cultist Warrior from the Wildwood -- and ran the map again -- this time it completed with no issues.

Not sure if it's related to necromancer spectre minions altogether as I play them often and almost always tend to experience this once or twice.

Specifically it seems to be related to the minions that the Cultist Warrior spectre spawns in this league, recognizing them erroneously as undead?

Anyways, hope this helps narrow down part of the issue. Not sure if it's game mechanics or Terrain/Area related but here you go.