CATACLYSM'S WAKE - Corrupted Nexus Boss won't spawn

My Endgame Questline is bugged.
"Cataclysm's wake - Complete a Tier 2+ Corrupted Nexus"
Once joining a Nexus map and clearing all rare monsters, no corrupted nexus/boss spawns. I just get the message "Map completed" like its a normal map.

Can confirm I am:
- Entering a Corrupted Nexus map (Red ring around the corruption image)
- Using a 4 mod waystone at Tier 2 and above.
- Explored the entire map and killed all monsters.
- Map objectives show "Map Completed" (Upon killing the last rare on the map)
- Have spent my 2 Passive points in the Atlas tree from the previous quest.

I have tried 3 nexus maps, all with the same outcome.
I tried with waystones T2/T3/T5.

Does anyone know a fix for this?