I have similar a similar issue. Multiple times, the room the stairs lead up to has ended up not having/losing the ability to be assigned a room type (residential/public/employees).
I managed to restore it previously, but gave up when expanding the house one tile in width. It was somehow fine without the railing along the top of the stairs, but when closing the railing (in the position indicated by the wall tool in the second picture) it would turn into an ordinary room without any coloured outline. I tried to do it in a variation of ways to see if anything worked, but the same thing would always happen - whenever that one tile long piece of fence was placed, there'd be that thunk with dust coming out from around the room, as if completing it, though the outer walls remained untouched.
Edit: restarting the game fixed it for me:
