Hello friends, I was having the same problem with the 3D stuck in a wrong MVO and this worked for me (for now):Go to a saved Plan View (section or floor plan) > change the MVO to default > select the stairs > F5 > (mine came back to normal) and then ...

So, I figured something out, but I still believe it is a bug (or i'm just dumb)If I go to a view with set MVO (fit for exemple to a 1:500 scale) chage to default and then come back to the 3D view, it works fine!But if I don't change the MVO before go...

Yes, i tried that too and nothing workedAlso updated my Archicad to build 5010 and still no change

No, the settings are set to go with MVO But even if it was so, the stairs in the other file is fine, it's something wrong in some viewing settings in this main fileEven tried to make new stairs with pre-made settings: