So it's been 4.5 years since I first reported this issue. At this time, it seems that mp3tag can neither write nor read the custom m4a xtra atom that windows / wmp is using to store ratings.
My first two posts in this thread cover the issue thoroughly.
Hopefully having one column for the rate atom, and one column for whatever the window's xtra atom is, will be a workaround that mp3tag can implement. It's similar to this issue:
What I would do with my mp4/m4a files, is use mp3tag to first delete any ratings that may be in the window's xtra atom, and then use mp3tag to copy all my rate atom ratings to that xtra atom. (Redundant but necessary to show the same rating value in both windows, and various apps)
I think it would be smart for mp3tag to include an option for users where mp3tag automatically keeps the two fields in sync on any given writes, and allows the user to pick which atom should be given higher priority. I would suggest the rate atom be the default.
Thx.