The Copper CItadel Completion

I just killed the boss in the Copper Citadel, but i forgot a rare monster in my map, and i can't go back to get him ! so i couldn't complete my map, idk if it's a bug or an improvement to do but i created a post in improvement section also

Same just happened to me. They need to add a checkpoint down there so you can go back. Crazy how it was overlooked.