Hi sneather, it's probably happening while MoI is reading the STEP file.

The way NX and also STEP format handles trim curves is a bit different than how MoI handles them - MoI handles trimmed surfaces by a UV space boundary in addition to the 3D edge curves, while NX/STEP can contain only 3D trimming curves. The main difference between these is on surfaces that are closed and have a "seam edge" in them like that particular one. If NX writes out trimming curves for a closed surface where the trimming curve crosses right over the seam edge, MoI has to cut those particular edges up into separate pieces where they cross over the seam (because UV space ends at the seam), and that can be a kind of delicate operation and it's likely to be what is not working quite right on that particular example. It could be exacerbated by things like a trim curve that kind of wavers a bit back and forth across the seam or things like that.

You might try using IGES format instead for a case like this where you've run into a problem - IGES files will often have surfaces in them that are trimmed by UV space curves, and those trim curves can be used directly by MoI's importer rather than needing to be split up at seam crossings. Sometimes IGES files can have different problems though so it's not a bad idea to ask for both STEP and IGES files both and try the other one if you run into a problem.

If there are only a small number of those problem areas, it's possible to manually repair those things by an untrim/retrim operation. To do that, select that face and use Edit > Separate to break it out into its own surface, then select one edge and use Ctrl+A to select all edges of that surface and hit delete to untrim it and recover the full underlying surface. Then use Edit > Trim to trim that surface with the edges of the surrounding pieces and then join the newly trimmed surface back in.

- Michael