This bug seems to be present on two houses I have built with the armoury wing. Sometimes when I walk up the stairs that are built on the outside of the wing I just drop through the stairs to the ground after walking half way up them. and if i sprint at the stairs to run up them then its even worse I just run right through them. I find the only way to successfully get up the stairs is to keep as close the the wing wall as possible and walk up them rather than running.

That happens to me, usually the first time I try to go up then after loading that area. All attempts after that, they work just fine.

Is this happening every single time you try to go up them?