Cylinder surface has gaps
Creating a Custom Element and a Cylinder surface sometimes results in a surface in the 3D View that is incomplete.
This could be because the Trimming Volume has the same radius as the cylinder. A surface can only exist inside the Trimming Volume, but in this example the surface is exactly on the boundary. FRED is simply unsure if the surface exists or not.
The solution is simple, the Trimming Volume needs to be slightly larger than the surface.