I built a simple closed shape, a kind of up-down stroke on top, and a convex bottom, then extruded it. the result is a sort of cylinder with concave dent on the side. I’m not sure yet, having not imported it into blender, but in Codea, the faces in the indented part don’t appear. I suspect they are present but their normals are inverted (because that part is concave?).
If this is a known thing, please comment. If not, I can explore further or provide pictures or whatever would help.