Party members sometimes get disconnected when going inside the portals to the following floors.
Whenever it happens, the doors to the next room do not open even when all monsters are dead.

2F - 1st Room


Boss Floor (Happening in this picture now ><)


The disconnection usually happens while they are on the loading screen for the next floor.

I'm not sure if the door bug only applies to this particular kind of disconnection or if it also happens when someone leaves the party during dungeon run.