I did it three times last night, never once glitched. I personally opened the door each time. I stayed in the general area where the 3 hobgoblins spawn in the second wave. I never went on the rocks other than to do a revive and jumped down immediately. I never went to the lower area, killing floor, until after the third wave completion. Didn't have an issue. Seems to only affect when all three people are out of potential melee range.