It's about the height...!
Staircase height was set to wall height + floor thickness, and it must be a little bit more to be visible on both levels.
Thanks a lot

Hi.
I created my own staircase using AOI, and imported it as on .OBJ into my project (I'm using SH3D v3.4). The thing is, I cannot see this staircase on the floorplan of the upper level. This also happens with models converted from .SKP to .DAE.
Could someone please help?