here is a weird one. Have Polysurface as Solid Using Clipping plane it properly Shows as a Solid and Rhino cap the Surfaces.
Make an Object of this Polysurface, now the caps are disappearing…
Here is the Polysurface clipped showing as solid:
It is a solid as shown here:
Now Make a Block out of the part:
Part now looks Hollow…
Although it still believe it is a solid…
Is this a Normal Behavior???