Hi Steve - also if you zoom in a ways on the seam areas of where the 2 tubes cross each other you can see why it's difficult for these kinds of things to get auto mitered:





You can see that there's a rotational difference between the 2 shapes there, the rotation would be a lot more obvious on something that wasn't a circular shape.

In some areas that rotational difference was small enough that it was ok, but in the ones that failed it happened to be a bit larger and if it's too large it significantly complicates the miter trimming mechanism. The mitering can only work if the surface pieces meet up in a more precise way, so usually it only works if the paths are 2D curves.

In the future I do want to add in a different corner mitering style which would to put in a slanted profile and then sweep to that slanted profile rather than doing an extension and trim type method, but sweeping to a slanted profile produces a slightly different kind of shape. You may need to do a manual slanted profile approach to make stuff like this work now though.

- Michael