Cital Map (Iron Citadel) unfinishable

Played the T15 Iron Citadel Map, killed the boss first and cleaned up remaining rare mobs afterwards. Map never triggered as complete, though the quest completion triggered.

Maybe relevant: the red skull symbol for the boss was still visible on the minimap even after killing him.

Screenshots of the cleared map and failed spot on the atlas:
https://imgur.com/a/1f39udt

I have the same issue. Already "bricked" two Iron Citadel maps like this.
You get the loot but not the completion and you cannot progress to areas further behind the citadel.

me 2