Vaal Foundry - Cannot go back in the map from the boss area

So, I missed a rare mob in the map.. and I went down and killed the boss. I didn't pay attention that the map was not completed so I portaled back to HO. I went back in to see a rare mob in the map.. There's no portal or door to go back to the area before the boss, so I can't complete the map now. This needs to be added. Thanks

Same here, no way to go back up