Can you provide a link to one of the song files which exhibits this behavior? We can analyze it at the binary level and find out what's wrong with it.

My guess is that there is more than one ID3V2 tag in the file, and that MP3Tag is writing additional tags to the file but not removing the old one. Another possibility is that there is some kind of additional text data appended to the file that isn't actually an ID3V2 tag or an ID3V1 tag.