Hi ppj, the corner mitering mechanism may have difficulty on paths that are swooping around in 3D rather than all in the same plane.

Mitering is done by an initial sweep along the main part of the segment, and then extensions of the surfaces.

When your path curve swoops and bends around all over the place in 3D instead of only in 1 plane, it is likely that the extended portions do not have a very clean collision between them anymore.

You will likely get the same kind of problem elsewhere, for example here I have opened your file in Rhino and tried the Sweep1 command in there:



As you can see it gets similar results.

Primarily for a mitered corner sweep you would want either a path all in a single plane, or something made up only of line segments, not something that has bendy parts and also not planar.

You may want to put in some rounded corners (trimming back and using Blend is one way to do this) so that you have one smooth rail instead of a segmented one like that.

- Michael