I stumbled in a strange situation, though I'm not fully sure it is a bug and not something I've overlooked.
I am in the basement of Asomond's palace. After killing all the hirelings and the special enemy (don't remember the name) I went back to the sewers to rest and do shopping.
Then I went back to the basement and killed again the respawned hirelings. When I attempt to climb the stair to next floor, Jerrat complains that we must clear the floor.