Hi Martin, so yes this has to do with the particular kind of mesh entity in your STEP file, the STEP "FACETED_BREP" entity type. This is a separate issue from what Rico wrote about above.

Moi v3 did load the "FACETED_BREP" entities but that got broken in v4 since the Jun-6-2018 v4 beta release which reworked the STEP importer to use multiple CPU threads.

This is the first time it's been reported in the last 3 years because it's kind of an obscure and rarely used area. I'll see about restoring the v3 behavior for the first v5 beta release.

Thanks for reporting it and sending the file!

- Michael