Sorry, I do not have any idea. Should copy just like any other tag. They do here.
You didn't change the setting recently, did you? What I mean is, these ratings are currently saved as tags in the file, not database only, right? Simply changing the setting from database ratings to tag ratings after having rated songs to the database doesn't go back and change all of the already database-rated songs so the rating is now in the tag instead. It only changes what will happen going forward for new ratings.