During shelling, we think the operation has been ok but in fact, shell has done a curve (on previous picture selected), but didn't use it to construct the internal surface.
It constructs a single surface which is unique, intern and extern.
Hi Phiro, the weird piece that looks like a curve is a surface but with a messed up trimming boundary on it which prevents it from getting a proper shaded display like surfaces normally have. If you select it and use Edit > Show pts you can see the surface control points and if you select its 2 edges and use Delete to untrim it that will recover the surface.
