Hi Steve - generally to get mitered corners on a sweep you need to have a planar curve for the path. Curves that are arcing around in 3D don't necessarily have the same kind of clean juncture between them where the pieces meet unless they come to a planar area in the region where they touch.

For something like that you may need to put in a profile section that's at a 45 degree angle in the original shape, and flow that and use that angled profile as part of the sweep rather than relying on sweep to miter the ends which instead involves intersecting and trimming the sections against each other which then will not work if they don't meet up in a planar juncture.

- Michael