This seems to have happened after the platforms patch. I edited my chez llama slightly to remove the arch above the stairs (if I recall). That fixed it for me at least. There's also "fixed" chez llama builds available in the Gallery.
Yes, there's a bug with stairs at the moment in all builds. It seems if you have open stairs that lead to an open upper level, the sims can't go up them. You can remove the railings along the stairs and upper level to fix it for now. That's what I had to do in my celebrity mansion to get it to work.
It's an annoying bug!
When open stairs are used the game creates an invisible fence at the top of the stairs to complete the room where tiles can be deleted to open up the stair well. It is broken so with a few changes the stairs will again. Replace floor tiles to the open area removes the invisible fence. Stairs are fine. The bug lies with the invisible fence.
Comments
It's an annoying bug!
Indeed it does. It breaks a lot of EA builds too, like the Narwhal Arms Club in Windenburg, you can't go upstairs to even use the bathroom.