Looks like there is purge problem with materials in Rhino 6.
Here is the file - it was an imported SKP model that came in with materials (Rhino-created custom Rhino materials).
Once all materials have been stripped and set to ByLayer, all original materials still linger in the Material Editor (after Purge Materials=Yes).
Not being able to get rid of unused materials makes the files messy - please fix !
MatPurgeBug.3dm (156.1 KB)