Stacked staircase bug

I placed a staircase to the basement underneath the staircase of an existing house, and for weeks (Sim time) there was absolutely no problem with it.

Now, out of the blue, my Sims get this weird bug where they walk from the basement to the top of the stairs, turn around and walk back down again. I've tried replacing the stairs with new ones (even used a different type, just to make sure), removing the railing, resetting the Sims and resetting the lot, but nothing has helped so far. I don't have moveobjects on (but I may have had that in the past), and I don't have testingcheats enabled standard (I do it manually and shut it back off).

Does anyone recognise the issue? And is there something I can do about it, or do I have to re-think my entire basement lay-out and place the stairs somewhere else? It's really annoying, especially since it all worked fine. All I did in the meantime is install one Store item (the Tree of Prosperity) and nothing else changed. I'm not using any mods in this game installation. I don't even have a mods folder installed.