Hi PaQ, I did a bit more investigation - it is a bug in the filleter which is deleting that "seam edge" of the back-side fillet during a post-processing step of filleting where it tries to remove what it thinks are unnecessary edges.

That edge is supposed to be a seam edge so it is not supposed to be removed. However, the surface offsetter did not do a completely good job of offsetting that fillet, there is a very small gap between the ends which is confusing the piece that is supposed to detect whether a given edge is a seam or not.

I'll probably be able to fix that up when I get a chance to look at it in more detail, but also you will can probably get some better quality geometry if you do the shelling first and apply fillets after that, rather than shelling something that has already been filleted. Although there is kind of a Catch-22 there because if you are trying to shell by thickening a surface instead of shelling a solid you may need to have a fully smooth sheet for the surface-thickener to work and not get messed up vertices.

- Michael