If I import the geometry from the file "Iron Core_TM_18_CoilUnit_Flex_CAD_160603.stp" from the following zip: https://www.tecnotion.com/downloads/iron-core-motors-cad-files-tm-series.zip

The geometry initially imports as a joined surface. If I change the object name and save the file it then randomly loses most of the geometry (leaving two small patches from the sides) and changes to a solid. Any ideas?

Thanks,
FDP