Hi okapi, yes currently MoI does not do the extensions and trimming that need to be done to automatically make mitered corners there.

It is something that I am planning on working on for v2 though, this along with some other sweep improvements will probably be one of the last things added right near the end of the v2 period.

To get the case that you show there finished up now before that, I would recommend using Offset on the curve, with the "Both sides" option enabled, then draw connecting lines to cap the ends and extrude the offset to get the result.

- Michael