I first was going to post this on the Grasshopper forum but then realized it’s actually a Rhino issue.
See the image below. The black curve is the original curve. The green curve is an offset of the black curve.
The blue curve is the same as the original curve but flipped. The red curve is an offset of the blue (flipped) curve. It creates extra control points in this corner on the flipped curve. Both offsets are at 1 mm distance. I’d call this a bug.
190408 Offset flipped curve.3dm (33.6 KB)