I would check to see what renovation filter is active in 3D to see if it different to the plan.
Also check your layers showing are the same.
Or check your 3D filter options as stairs may be turned off there.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11