It doesn't matter where it happens. This can happen at every staircase. If all players except for 1 are waiting at the staircase, and the 1 that is missing dies, he is dead. Doesn't matter if there are infinite lifes. We have had this topic before and that was the same problem at another place.
This also happened to me, and I think Myran already fixed it in an upcoming patch.
So as long as it isn't fixed. Don't wait at the staircase, wait in front of the staircase. If you are 3 or more players, 1 of them can enter the staircase, because then everyone knows, that people are waiting to enter the next floor.
But that the player respawns in the next act is new to me ^^