I am having this issue with sub curve where not all halves are on the same side.
it is supposed to be like the image below:
unnamed.gh (44.1 KB)
it is supposed to be like the image below:
unnamed.gh (44.1 KB)
the problem might depend on not all the lines sharing the very same direction, maybe some of them are flipped?
the best thing you can do is posting your .gh file after internalizing all the referenced geometries