Can't Change Artwork for Home Videos in iTunes

I'm trying to organize our family home videos from the last 15 years or so. I've loaded about 100 or so vids into iTunes and categorized them all as Home Videos. I originally used a single family photo as the artwork for every video, which worked well enough. But when we look through our vids via AppleTV, seeing the same thumbnail over and over again makes finding a specific video a little tiresome. iTunes does automatically generate a thumbnail from within each video, but most of the time the image is distorted and doesn't give you a good idea what the video is actually about, so I decided to take a screenshot from each video and using that as the artwork instead.

That worked just fine for the first 3 videos, but when I did the 4th video, it applied that same image as artwork to nearly every remaining video (a handful of videos retained the original family photo artwork).

Now I can't change the artwork on ANY of them. I've tried deleting the artwork from each individual video and I've also tried selecting multiple videos and deleting the artwork, but it's still there. I've closed and restarted iTunes and also restarted my Mac and the problem still remains. I can't edit artwork for any of these videos except for those first 3 that didn't give me any problems. I've even tried deleting the videos from my iTunes library and then adding them back in, but nothing changes.

What's really weird though is that this seems to be a display issue/glitch only in iTunes. When I scroll through the home videos on my AppleTV, the correct artwork is showing.

Music/Movies/TV Show artwork appears to be unaffected.

iTunes 12.8.2.3

High Sierra 10.13.6

Any suggestions for a fix? Thanks.