Duplicant mopping through a wall

Originally there were just four tiles of floor between the pool of polluted water and waterlock, the wall to the right of Otto wasn't there. The pool started overflowing into the waterlock, so I gave my dupes order to mop the floor (all four tiles) and at the same time to build a wall separating the pool.

I don't know what exactly happened but I assume Otto started mopping the rightmost tile and while she was at it, some other dupe built a wall there which moved Otto one tile to the left while the "place of mopping" got moved one tile to the right. So now Otto is mopping top layer of the pool while standing safely on dry floor.

Unfortunately the situation doesn't survive a save/load cycle so I'm not attaching a save.

The job is locked on the gray mop command now embedded in the wall - if I cancel it, Otto stops mopping.