(windows) command line m4a tagger?

[simoncn]

I think destroying valid tags is a very serious bug and you should definitely report it to the author of Mp3tag.

I don't have many mp4 files myself and I am not familiar with how different taggers treat multivalued tags in these files. Perhaps other users with mp4 libraries can share their experience.

In practice, the way these issues are resolved is by users such as yourself coming across problems and reporting them to authors of servers or taggers who will then (hopefully) fix them by implementing the spec more correctly and completely, which benefits all users.