Musicbee does correctly save multiple genres as separate instances of the tag (Tag Code TCON in ID3v2, GENRE in VORBIS).
From what I have seen, this is true for Vorbis, which allows multi-field instances, whereas this is not the case for ID3... which as SandaruLJ stated is infact a NUL separated character string.
i.e. If I query TCON on an ID3 field w multi value genres via Mutagen (https://mutagen.readthedocs.io/en/latest/
) it does show <0x00> separators in a single TCOn field... whereas in MusicBee it does indeed look / display as if it were multi fields [which, afaik is not possible with ID3 either]
Would be good to get an official statement on this, as I am not a data/hex specialist or programmer...