This is by no means a definitive answer but my take on it.

Due to licensing/copyright issues, especially with album compilations, the song/album may be removed from the music service. The greyed out songs appear more evident on older created playlists. When this happens, I believe Spotify ‘replace’ the song in the playlists with another link to the song so it continues to play on Spotify without any problem.

I’ve also noticed with some songs the artwork when playing has changed from what it was when the playlist was created. For instance, songs that previously used the compilation artwork when the track was playing, can now display the actual song/album artwork for the artist instead. Whether this is purely a change of artwork or more happening behind the scenes is up for debate, but to me it seems the greyed out songs are the ones more impacted by a change of artwork.

Such changes don’t always appear to filter through to Sonos successfully, hence the grey tracks that appear. However, based on what I’ve read previously on this forum, Sonos can only display what the music service provides. Therefore, in this case, Spotify is proving duff information to Sonos (ie. invalid links to the song) and the problem is due to the respective music service rather than Sonos.

Yeah I agree that is the most probable answer but would love to know more about the disconnect and why it occurs when it comes to the updates not propagating to the Sonos app.