Endgame Corrupted Nexus not counting towords quest / dosent spawn Corrupted Boss

I had a corrupted Nexus map. Map Layout Forge. The final boss was the Colossus. After killing the boss no corrupted Boss spawned, which lead to the quest not completing for clearing a Tier 12+ Corrupted Nexus.
The maps in the area are defenetly cleared and after reentereing the map it says that the monster Level is 79 which would line up with a tier 15 map.