If it becomes a preference setting, then expect this Forum to be flooded with support questions. Users don't normally RTFM or normally read the release notes and thus it's WHY isn't MP3tag not doing this anymore, it used to work etc etc etc
Not feasible IMHO. Leave it to Florian he will find a way 
I've just released Mp3tag for Mac v1.9.10 with a fix for the issue where the MP4 gnre atom for GENRE used a type identifier that was incompatible with Apple Music so that "Unknown Genre" was displayed instead of the actual genre.
To be fair, Mp3tag used the correct type identifier, but Apple Music expects it to be unspecified to pick up the genre. This issue was introduced with v1.9.7, where lots of the MP4-related code changed.
To ensure that the genre is written in the correct format, you can change the genre to something else, then save, and then change it to the actual genre you want to use. This way, the genre atom is rewritten by Mp3tag in the correct format and not reused and simply copied over when saving.
Thanks to everyone involved in hunting this issue! I hope that everything now displays fine in both Apple Music and MinimServer.
Works for me. Great stuff Florian, what a quick fix that was!!!
Thank you for confirming the fix and for reporting the issue in the first place!