February - last edited February
It's 2019 and this still hasn't been fixed.
-edit- sorry, the first post didn't show up so I figured something went wrong...
I’ve just been tearing my hair out at this and the stretching method didn’t work. The worst part was that I had 2 sets of interior stairs against exterior walls but the wall trim only disappeared on one. I compared the two and found a fix!
1. Before placing your stairs, use fences and walls to create the “stairwell” e.g 3x1 space for a standard wall height, single staircase.
2. Select the “room” (stairwell) and remove the floor.
3. Place the stairs.
Hope this helps.