Yes, I observe the same thing with .NEF files (not others, I think).
It seems you have to clear the "Artist" tag to prevent this.
Using ExifTool:
/usr/local/bin/exiftool -Artist='' -ext nef <folder path> -r -overwrite_original -m -P
Not sure this is due to PhotoMechanic. Maybe the "Artist" tage set by the camera itself?