[HeresJohnny]

While trying out one or another thing I played with some tags and rescanned them to see their result. Looks like indeed FLAC files with multiple artists and genres are handled perfectly without itemseprator. Almost.
The exception is - EMBEDDED CUE SHEETS. Man, I really thought it was a good idea back in the day when I ripped my CD collection to single files with cue sheets. That way I'd perfectly preserve gaps and other aspects. After ripping, I've created playable albums with tags by activating "Enable embedded cue sheet on this file" in foobar2000. After that, the contents look like and are treated like separate songs in the foobar library and playlist for all intents and purposes.

Unfortunately, those kind of files behave differently when their tags are read in KODI. The crucial tag fields genre, artist, album artist are not separated properly but instead presented as a single comma-separated string:

Example:
VORBIS TAG genre:   Anime; World; Soundtrack => Split correctly into their separate genres
EMBEDDED CUE SHEET genre:    Anime; World; Soundtrack  => Listed under genre as    "Anime, World, Soundtrack"

Hopefully this can be fixed.