Okay ... it is entirely possible that the files you download use a semi-colon to separate multiple artists and do not encode them properly for ID3v2.4. ie. they incorrectly place a semi-colon in the encoded data.

Send a dump of the downloaded files before you save to them. I do not need the saved files as they are encoded correctly.

If your player always uses a textual ; then you cannot encode the files correctly to represent multiple items in a field. This includes FLAC and all ID3 based formats (mp3, dsf, dff, wav, aiff, etc.). MPEG-4 files can be configured in a variety of ways. The most typical is that multi-item encodings are ignored in which case the delimiters are written to the files.

If you always want to write the semi-colon text to the files then manually enter the semi-colons ... but DO NOT set the multi-item delimiter to a semi-colon.

If you change the multi-value delimiter back to ;;; and you manually use a single semi-colon, the files will be written as you wish. Of course any player such as Audirvana, Roon, MinimServer, etc. will treat the fields as a single item. ie. not recognizable separate entities.

The issue is that purchased/downloaded tracks might properly encode the multiple artists in which case you'll have to manually change the ;;; sequence to ;. It's been my experience that purchased tracks typically use a textual representation as they don't care if you can identify the separate names individually.

After changing the multi-item delimiter back to ;;; simply saving the files will restore them as you wish. You can do a File>Save Selected Files>Always to force save them.