“””””” Sometimes the curve needs to be closed and selected, but sometimes it works on an open curve, selected or not selected, or else "calculation fails".

For some curves it leaves behind a partial circular arc, sometimes a gap.”””””

Sounds like projection errors with the setup of the curves….

3d curves? Normal trims and booleans can do that when z values (or depth values) are introduced to the projection.

Can you make a file that does this?