The current code doesn't include selection boundary points in non-
selected curves after separation, causing some segments to be lost.
Legacy curves do include those points. The second issue is with cyclic
curves. The current version leaves the separated part cyclic, though
the more intuitive behavior would be cut off the result. This is also
different from legacy curves. This commit fixes both issues.
Pull Request: https://projects.blender.org/blender/blender/pulls/135035
45a2634917