Multiple tags with same name are ignored
Where there are multiple tags with the same name (e.g. COMPOSER) in a FLAC file, only one is shown when the metadata of the file is inspected. The tagging recommendations for these files explicitly say that in such cases each composer (but also artist in case there are multiple contributing renowned artists) should have its own tag. I have tagged my files that way and only one composer is shown.
I do seem to remember that previously (don’t remember the version) did show all composers, separated by a semi-colon.