Itunes and artist, albumartist and album sort tags

Hi, I updated the artistsort and albumartistsort and albumsort tags in MP3tag on some M4a tracks.

When those tracks are loaded into iTunes it does not see these sort tags and defaults the sorting to greyed out values that are the same as the non sorted versions of the tags ie artist, albumartist and album.
If I then update the 3 sort tags in iTunes and go back to MP3tag I see the 3 tags appear twice in Extended Tags (see screenshot) with exactly the same names (and data).

Screenshot 2022-01-26 151839

This is not an issue with MP3 tracks but is happening on M4a tracks


what happens if you remove duplicate fields (with an action)? Does iTunes then see the sort-fields?
Also, something that is a button to twiddle and tweak: which language have you set in File>Options>Tags>Enhanced? Even though this should only work for comment and lyrics, perhaps iTunes uses that as well and expects "eng"? But this is just a wild guess.

This is only relevant for ID3v2 tags.