As it is so problematic, use the Freehand and Brush tool and start close to but not touching the line and draw a closed (filled) shape.
Place behind the original and select both and Add Shapes.

To remove from the original, all od the above but Subtract Shapes as the last step.

The explanation I have in my head for the ~-~ use is it is all down to the winding direction of the added path.
I find it difficult to explain though, hence the alternative.

Acorn