Hi Alex, currently MoI will only attempt to do mitering on a one rail sweep if you're using 1 profile.

Multiple profiles is a pretty different case because with the profile curves constantly changing in shape, the miter is not going to be a planar cut at the juncture point even when the path is a polyline. In the single profile case, with a polyline path the miters are planar.

I was able to tune up MoI's mitering method so your case will work in the next release though, thanks for reporting it.

- Michael