Finished Duncan but gate is still closed

I have finished Duncan's dungeon as a tank.

I didn't receive rewards from Veth, Quests' List shows it is finished though.

But when I returned with henches and heros, the gate of Duncan was still closed.

I was dead in front of Duncan for signet of sorrow spamming and kept lying dead until Ducan fell.

My guess is: This is a bug or this happenes because I wasn't alive when Duncan died.

Did I figure it right? Answer to me please.

You can only kill Duncan one time per clear. After you kill him that one time, you have to clear all the other bosses first before you can kill him again. Even if you didn't take the quest reward.

I figured it completely wrong.. Thank you for the answer.