Hi Rico,

> Oh dear.
> While the fillet works fine (with rebuild of joined curves and 2-rail sweeps, yay) now the fillet operation
> has decided to do an unwanted "cleanup" of the rest of the surface.

It's built into filleting that it tries to remove "topological edges" from the model if possible, which means it will try to merge 2 coplanar plane fragments into a larger plane.

In MoI v4 I can see about disabling that but for now there isn't any way to turn that off, sorry.

- Michael