I never saw this in my game. I built with platforms, but not sure if there was any stairs object above that room.
Anyway, I have seen other objects from the beneath floor, like a room divider statue wall thing - I could see the "foot stamps" from the room below. I think that was before e even got platforms, though. Maybe t has to do with the alpha/transparency settings of an object? I also saw this effect on some CC where the object's kind of shadow bleeds through EA objects. I had those CC objects for a long time, but noticed the problem 1 or 2 months ago. We had lots of base game tech changes recently, like the windows/doors cut out, and the platforms. Maybe even some EA objects (like your stairs) need adjusting by EA to go well with those changes?
Well, that did not help you much, sorry :)