Hey @mikkelhk
I started replying, but I note that you've discovered something about .m4a.
But... it actually does still work for me. I don't have a lot of .m4a files, but I found some that had multiple composers, and they were written to the files with that setting.
I wonder if it's to do with the ID3 version? In the Advanced settings, you can choose to use ID3v2.4 rather than ID3v2.3 - I'm using 2.4. This only changes if you tell foobar2000 to remove all tags. And then there's a setting about which tags to write if the files are untagged - I have it writing ID3v2 and ID3v1.
Also, take it or leave it, but I've got some albums which have "Composed By" as the credit, so I've adjusted the tag mapping to be:
$flatten($multi_if($multi_add($any($multi_strcmp($sextend(%<<TRACK_CREDITS_SHORT_ROLES>>%,%<<RELEASE_CREDITS_SHORT_ROLES>>%),'Written-By')),$any($multi_strcmp($sextend(%<<TRACK_CREDITS_SHORT_ROLES>>%,%<<RELEASE_CREDITS_SHORT_ROLES>>%),'Composed By'))),$multi_if($put(aj,$sextend(%<<TRACK_CREDITS_ARTISTS_JOIN>>%,%<<RELEASE_CREDITS_ARTISTS_JOIN>>%)),$joinnames($put(an,$sextend(%<<TRACK_CREDITS_ARTISTS_NAME>>%,%<<RELEASE_CREDITS_ARTISTS_NAME>>%)),$get(aj)),$get(an)),))