Hi Rudl, you did not do anything wrong - that is a bug in shelling in the geometry library that MoI uses.

The geometry library assumes that each vertex from the original object will match to one new vertex in the output object. But that kind of 1-to-1 mapping is not actually what should happen in these kinds of cases, it's something more like the pieces will diverge away from one another in that kind of situation and there should be things like new edges introduced into the result instead of a single vertex to single vertex output.

So MoI's shell function is not able to properly deal with that situation, for something like that you may need to construct the offset by using Edit > Separate on the original shape to separate them out into individual surfaces, then offset those and figure out how you want to intersect them with each other.

I have been asking the authors of the geometry library that MoI uses to update this area of the library for quite some time, and they have told me they have had someone working on it but that it is still in progress.

- Michael