I have tried FLAC with exactly the same strings, converted to MP3 (yes, with ID3v2.4), and MP3 contains everything (checked in MM5 - Properties dialog - Custom tab). Are you sure you use the last MM5 build? Where do you check MP3 tags?
In case you reproduce it consistently, could you upload me affected FLAC file (for which converting in MM5 to MP3 loses tag) and PM me link?

[quote=MiPi post_id=474522 time=1605083462 user_id=27459]
I cannot reproduce, what are your exact repro steps?
I tried to take FLAC, tag it with pairs of all three extended tags (in Properties - Custom - Extended tags, two lines with each sort tag), converted FLAC to MP3 and all 6 tags are saved (checked in Properties dialog of the MP3 file).
[/quote]
Did you also use ID3v2.4 tagging for the MP3 files, as I do?
It happened i.e. for ARTIST=Queen; Elton John and ARTISTSORT=Queen; John, Elton.
The ARTISTSORT entry for Queen is necessary to correspond in sequence.

I cannot reproduce, what are your exact repro steps?
I tried to take FLAC, tag it with pairs of all three extended tags (in Properties - Custom - Extended tags, two lines with each sort tag), converted FLAC to MP3 and all 6 tags are saved (checked in Properties dialog of the MP3 file).

Sorry to open this issue again.
Though it is obviously resolved for COMPOSERSORT, it's still the same problem with the other two.
After format conversion only the last value is copied. :(
Regards
Peter

OK, makes sense, added as https://www.ventismedia.com/mantis/view.php?id=16030

A big thank you first for including sort tags in format conversion as I use them much.
Unfortunately if there a multiple values for ARTISTSORT, ALBUMARTISTSORT, COMPOSERSORT (occurs very often) only the last value appears in the copy, say from FLAC to MP3.
Should not be so difficult to fix cause for ARTIST, ... tags with multiple values everything works fine.