I hope you can find a fix for this issue as i do have loads of m4a files, many of my files are itunes purchases.
Sometimes i do tend to go back to enter tag on old tracks and by the time I write the tags to files, it removes all tags that i previously added to the label field.
As a work around i have now mapped to add the tag to comment fields as well, but i use my comment field for other personal tags that i don’t necessarily want to include in my global tag list.
Serato doesn’t actually store remixer in the m4a file, but in the database. The (unofficial) remixer ID3 tag for m4a is an iTunes tag that Serato seems to ignore.
So writing tags to file will not work for M4A files and the remixer field and then reloading tags in Serato.
What does work is to sync to Serato, then Lexicon puts remixer in the Serato database.
From my testing here, it seems Serato doesn’t write remixer to the m4a file but only the database. Basically, Serato doesn’t do m4a remixer field, read or write