Yesterday I experienced my first case of received multiple (two) genres from a well-respected source, namely Discogs (received via Mp3tag). The received Genre metadata looks like "genre1, genre2". It looks the same in the Extended Tags dialog (not one-per-line, as I was half-expecting).
In Foobar (the cut-down macOS variant), in the Properties dialog (right-click the media item) it displays the same string: "genre1, genre2". At the bottom of the dialog it advises: "Delimit multiple values with ;". Since there are no quote marks around this delimiter (in the advice) I assume it is just the single character ";". But I have no way of being certain about that.
So maybe space characters (in the metadata) are regarded as "invisible" by most apps? In which case there is no distinction between the aforementioned ";" or "; " (comma-space)? Advice on whether this is true or not would be welcome.