@fattybeeow Don't think you need a "room" to see furnishings below. At least, I haven't noticed it in my builds (re-builds, really, because Stairs and, of course, Sims.) Of course, in a lot of my re-builds, I had to put a fence on the "third" floor above the stairs to replace deleted ceiling. (Fortunately, you can delete fencing and the ceiling is still there.)
I'm still wondering if it's that lot in particular because your other lot is working fine.
One other thing that can make a lot and stairs wonky is platforms. Even if you just placed one and deleted it. There are visibility issues with platforms as well.