I have successfully been able to recreate this issue with our office template. Download link below.
You will see a dummy house with a stair next to it made from the new stair tool in version 21. If you go to the 3D view you won't be able to find it. The only way to see it in 3D is to select the stair in 2D, then hit F5. As soon as you click F5 once more to bring everything back, the stair disappears in 3D.
You can also select the stair and another object, say the wall, and hit F5, The wall and the stair will appear in 3D. But again, if you hit F5 once more, the stair disappears.
https://www.dropbox.com/s/szksex9w9elcnc3/TEMP.pln?dl=0
screenshots attached for explanation