Hi ppj, here is a kind of more generalized example to try and explain why swoopy bendy non-planar paths and mitered corners do not really go together so easily.

Here I've made an shape that has a more pronounced non-planarity to it so you'll be able to see the kind of result a bit easier:



Building a sweep from that:




If you zoom in on the juncture area a bit, you can see that swoopy paths just do not produce the same kind of uniform miter cut where they touch one another like you would get on a planar path:



In fact in this case the intersection curves between the surfaces look like this:




The general problem is that a nice mitered cut corner needs to have a lot of regularity throughout the area where the pieces are touching one another.

When you have a path that starts to bank and curve around the instant that it leaves the corner, it means that there is not that regularity other than in the tiny area right at the corner.


Another thing you could try to get something more like what you want would be to try and get some line segments in there for the corner areas where pieces are touching, when a sweep follows a line segment in the intersecting area as it leaves a corner, that provides a lot of regularity and matching between the pieces....


Anyway, your shape is not as non-planar as the one I show here, but it is still nevertheless nonplanar and so you will be getting the similar kind of effect as this case, just to a lesser degree that is harder to see.

- Michael