left a note on the edit, but I figured I’d explain here.
as said in my note, I do agree that if a thumbnail was created, it definitely belongs on a release (actually, even if it was automatically selected, I think it’s still good to have). however, I wouldn’t add this art to the wrong release (in this case the Spotify/Deezer/Apple Music release), as that release never had that piece of cover art
(I’d hold off on acting on the below until a decision comes out of the discussion of STYLE-2428 for adding lyric video releases)
I would add this art to it’s own release for the lyric video instead, as it was actually distributed with that piece of cover art (via YouTube’s thumbnail system)
cover art in MusicBrainz is primarily meant to identify releases (tho moreso for physical, it also often applies to digital). by adding a YouTube thumbnail to a Spotify release, it might become more difficult to properly identify releases such as these, thus damaging the integrity of the database
complete sidenote, I do appreciate you joining the push for music videos~ I’ve been trying to get a better place for them for at least a year, maybe two, and we’re finally getting some good and proper discussion with the style leader~ 
