Hi Steve, this is a limitation currently in MoI's sweeper.

It takes extra work to handle mitering the corners and I haven't had time to set that up for version 1 of MoI.

It is something that I intend to add though.

There are various work-arounds, for example use 2 rails with corners for a 2-rail sweep instead, but the easiest for now is to use Rhino for that kind of sweep.

- Michael