Quickfield tech support is actively looking into the problem (all credit to them - they have been impressively responsive) and they have suggested that the version of ST-Developer (from STEP Tools inc) apparently used by Rhino in the generation .stp files may be a factor here (this is simply a suggested possibility). The problematic .stp files were generated using ST-Developer V15 (visible in the text of the file header). Files generated using Quickfield tech support with Rhino 7 seem to use ST-Developer V16.5 (again from the header text). The latest version of ST-Developer (from their website) seems to be V18.
Does anybody know if there is any way to upgrade this aspect of Rhino 5 to the latest ST-Developer as a possible fix?
Also, this old thread is discussing a very issue relating to problematic .stp file generation similar to the problem I’m seeing. Once again a Solidworks import/export dance seems to have been a workaround.