I’m not sure if this is a bug, limitation, feature or something else, but i just realised, that you can make hollow solids in subD using OffsetSubD - probably due to them being mesh-based. What’s more interesting, is that it carries over when using the ToNURBS command, meaning that you actually get a valid, closed polysurface, but with two disjoint shells. Clipping planes even show them correctly and it’s not tagged as a bad object.
If you explode and re-join, it “correctly” becomes two closed polysurfaces.
I don’t know if this “feature” ought to be changed, since Rhino usually don’t allow this type of geometry to be created.