I left Duriel fight after dying to repair my gear (I don’t believe I moved, I think I teleported directly from where I respawned). The portal appeared to be too close to the boss room and had perhaps spawned Duriel while I was gone? As I returned through the portal, I dashed through the boss door which immediately clouded, but I noticed something was off as one of the 2 Duriel shadows had already spawned. I noticed Duriel’s boss icon on the minimap, but he was stuck underground, and after dispatching the shadow that had spawned, I appeared to be soft locked with the only way to leave being exiting the game, forfeiting my spawning resources. I am unsure if it is more portal placement or the fact that I immediately re-entered the portal and dashed through the room entrance, but I will try to update if I encounter the bug again, with perhaps waiting before entering the boss room after reentering the portal to see if allowing more time between the steps allows the fight to start properly.
I just had that happen to me 
for the 3rd time. So annoying. This was on Console (ps5)