I think Kaoru needs to do a few repairs, the filth is seeping in!
In the corner of the stairwell between the first and second floor (i.e. here):
the game starts stacking filth upon you. It appears that it thinks you/part of you is on the outside of the wall and on the ground, as there’s filth directly outside that corner - you’ll notice that part of my character’s arm seems to be in the wall.