I've experienced this with many different parts imported from many different sources. For example, I've downloaded a number of STEP files from 80/20's online catalog of fittings. Each time, I convert these STEP files to Solidworks parts. I use the automatic feature recognition, which usually works. However, once that new part is incorporated into an assembly, the mates initially work just fine -- until I close and re-open the file. Then all of the mates fail at once. I have read somewhere that this may be due to the Surface/Face ID changing, but I have gone through and manually given each face a unique ID, and it still breaks.
I have also verified that my converted SolidWorks part files are fully defined.
Is anyone else experiencing this behavior with imported STEP files?