Album art not showing in library, but showing on mini-player

I have a modest music collection that I have kept on iTunes and now Apple Music since my teens. Over the years, I have taken time and care to add album art to each track. However, since a recent laptop transfer, I have noticed that a lot of my album art is missing from the Apple Music library. This is despite the album artwork being found in the folder where the music is stored. So, the album artwork does exist, the library just isn't displaying it.

What makes it even more puzzling is that some tracks do show the missing album art, but only on the mini-player.

Can anyone suggest a fix for this? I would much rather avoid going through my collection again to reapply album artwork.