Hi

The reason is the point structure of the surface. Because all point rows meet at a single point, you actually get 5 points at the same spot.
This creates self-intersection when the surface is offset into the concave form, and probably an extremely short edge when it is offset the other way.

Try using NetworkSrf and make a rectangular shape which you then trim with the round boundary. See my attachment. It isn't precise, but its possible to make it fit quite exactly to your desired shape.

I would avoid single point heaps as far as possible to avoid problems like this.


Regards,
Fredrik