Nameless Seer didn't spawn in map

- selected Nameless Seer lantern mod on last pack on list (very low pack density)
- cleared whole map until 0 monsters remained
- checked T7 Promenade 10x fully, called my friends to help finding Seer
- we didn't find it

My suspect is that monster packs with Seer mod didn't spawn - icon looks like a Merveil siren, which I think I haven't seen.

Bug reference number 2514004981

Hope you will be able to confirm whether it's possible bug, or something else. Kinda bothers me if I missed him or not :)