[DaveBlake]

I think you have got it as I believe the problem correlates with genres with albums which I could not associate with Musicbrainz IDs. They still existed, because I did not retag them (my fault!). But the tag structure for those albums is pretty old and outdated, which is probably why they were "misidentified" as compilations, and therefore not shown. Would also explain why I did not have the problem with other obsolete genres, like "Chansons", where all albums were retagged and re-genred with Musicbrainz IDs / Picard. The "Chansons" genre disappeared right from the start... :-)

A small correction (I am on a single-handed mission to banish the confusion over what is a compilation, and how Kodi handles them).

The old name for the "Show song and album artists" setting is badly misleading, and Kodi did not misidentify your older albums as "compilations". In the library those songs had both song artist and album artist (not VA), and they were not flagged as compilations. But somehow all the songs have 2 genres ("World Music" and the other e.g. "Spanish Folk") in song_genre, but the album_genre only has "World Muisc".

I can't deduce how that happened. And the situation of the genres not being empty (because songs had that value), and yet because you we set to see only album artists when you clicked on the old genre you so no artists, only made it look more confusing.