Thanks for this super-rapid response!
I'm not sure exactly when it happened, but let's say within the last few days as I was uploading music files from a batch of newly acquired CDs. I should mention that I use iTunes primarily to access to my collection, 99% of which is music uploaded from CDs. Not exactly in keeping with Apple's current business model, but they get plenty of revenue from other apps on my iPhone.
Pardon my limited knowledge - can you tell me what a snapshot is?
Also, a few clarifications: I use iTunes and am avoidant of Apple Music (based on all the user concerns voiced elsewhere, plus I'd be paying a monthly fee for access to a collection that did not come from the iTunes Store). And I'm unsure that I can access the iTunes Store to get album artwork without changing from iTunes to Apple Music. Also, it would be tedious - but not impossible - to download several hundred albums' art with Get Album Artwork, if I can even access the Store from iTunes.
Finally, a new development - but I'm unsure how to take advantage of it. On a different PC that I use for backup, I discovered a version of my iTunes folders with the album artwork fully populated and visible. It's an odd configuration - the iTunes Library file is on the C: drive, while the iTunes Media/Music files, Album Art folder, etc. are all on an external D: drive. Is there a way that I could copy these folders and files to my primary PC to resolve the issue?