Yeah I see now you've got a helix section and then a tapered helix section - those will have a different tangent at the spots where they touch each other so that means there is a sharp kink between them. Probably the sweeps generated along those different sections do not totally cleanly intersect with one another which would be needed for the mitering to be processed.

In the future I want to add a different miter mode for 3D swoopy type paths where it will do something more like put in an angled profile piece at the juncture areas instead of doing a true miter with the extension and intersection methods, that might be what you would want here.

- Michael