It is almost certainly related to this particular build. Sometimes they can climb those stairs, and sometimes they can't. When they are stuck in the basement, and I go to Manage Worlds and come back to my lot, they now can climb the stairs. But I am trying to create a new build, and I want it to work normally.
What could be causing this problem is that the ground level of this building is raised a bit, so that smaller stairs could fit from the room above to the basement.
I can try testing this without one mod I am using but not for this save game, and on another lots, but it would take days and very long time, because this problem happens just sometimes.