So this issue is still in the game where in a any difficulty, and taking either a long or short time to kill King Rastakhan, the door up the stairs fails to unlock.
Vrakthris in another post says it’s not a bug but is intended due to “things not triggering the way they are intended.”
I just spent ~20min meleeing Rastakhan down as a warlock and still the door is locked.
After ~3 years and this raid not being able to be completed, it’s time to do something about it.