Because Sort fields are not supported yet, [url=https://www.ventismedia.com/mantis/view.php?id=2915]it is planned for version 5.1[/url].

PS: As I don't use iTunes since many years, the 'non-standard' tags originate in reworking the tagging with mp3tag.

[quote=MiPi post_id=476877 time=1610654600 user_id=27459]
I have analyzed the file and the problem is, that these tags are not extended tags supported by MediaMonkey. They are even not part of ID3 standard. MediaMonkey reads/writes extended tags from standard TXXX frame, where any user defined tags can be stored. But the file contains non-standard tags TSO2 and TSOC, these are not supported by MM. It seems, iTunes started to use these tags for some reason, no matter that they are not part of standard.
[/quote]
As for MM5 the favorized external DB is MusicBrainz please have a look here:
https://picard-docs.musicbrainz.org/en/appendices/tag_mapping.html
and you'll find these ID3 tags too.
Also other tagging apps like mp3tag (https://help.mp3tag.de/main_tags.html) a.o. use them and not TXXX tags, especially for the sort tags.
Here's how MinimServer handles the mapping: https://minimserver.com/ug-library.html#Tag%20mappings
So why MM is going this proprietary way?

I have analyzed the file and the problem is, that these tags are not extended tags supported by MediaMonkey. They are even not part of ID3 standard. MediaMonkey reads/writes extended tags from standard TXXX frame, where any user defined tags can be stored. But the file contains non-standard tags TSO2 and TSOC, these are not supported by MM. It seems, iTunes started to use these tags for some reason, no matter that they are not part of standard.

Which tags are not read, could you send me (to PM) such file, which contains tags not read by MM? Thanks.

For mp3 files extended tags (like sort tags) are not shown under custom tags though they are present, except if entered (again) in MM. :(