We were doing it on normal. So perhapes it is difficulty setting related. It makes logical sense after all.
Perhaps if you get all the keys to unlock the doors as well. My group noticed, on normal, that if you unlock all the doors, his DR is reduced compared to if you do not.
Also with our group, it was noticed that holy was helping to stop the boss's regen. Perhaps it is a case of the regen stopping is based about difficulty as well.
Visty, what difficulty were you doing it on? We may all be correct, and it is difficultly setting dependent.