Hi neo, yeah that offset problem is definitely a bug.

Unfortunately it does not look like an easy one to fix, it will probably need to wait until the authors of the geometry library that I use are able to improve it. That may take a while.

In the meantime as workarounds you can either offset the original surface before trimming it, or you can split the surface into 2 pieces, and then each individual piece is able to be offset and then joined afterwards.

I've attached the split up version which you can use to finish the part.

- Michael