I had exactly the same problem the other day. This is what I tried in the beginning (but withoug success):
- change from MTP to mass storage device (i.e. manually save tracks to sd card)
- change conversion type from MP3 to AAC
It always ended up the same, showing scrambled ID3 tags. However, some folders could be moved smoothely to both internal storage and sd card. I then realized that everything works fine when I let iTunes do the labelling. I can manually move these files to either storage and everything is displayed as it is supposed to be. Apparently the built-in player can’t handle tracks whose tags were manipulated manually.
I personally would NOT use MediaGo from Sony as this program collects shiploads of private data which is somewhat the exact opposite of the main idea of using a Fairphone. Just read MediaGo’s privacy disclaimer and you’ll keep your hands off.