Sim gets stuck under the stairs

I tested in my game, and I cannot duplicate this issue.  I built a structure that resembles the setup in your picture and sent a sim up the stairs.  Went into build mode when he was halfway up, bought some furniture and went back into live mode.  He continued to climb the stairs to the upper level.  Sent him back downstairs and went into build mode again as he was descending, and knocked down some walls, to see if structural change could cause the problem; it didn't.  What specifically are you doing when you go into build mode and this happens?  Perhaps I could recreate the issue if I had more information...