Did you check the linked thread
The Windows Explorer has the habit of writing the rating into a private XML tag (at least for MP4s) and if you do not see any ratings in MP3tag then something similar has happened to those files as well.
I just checked with an MP3 file and set 3 stars in the windows explorer properties dialogue and that appeared as expected as
So you have to find out why your files do not show this field.
Perhaps it helps to turn off to read APE tags.