I download Let's Play series off of Youtube and sort them to watch through Jellyfin at a later time. I also download the thumbnails. On June 9 I upgraded the Jellyfin server from 10.9.1 to 10.9.6 and thumbnail importing broke somewhere inbetween those two versions.
As you can see, episode 25 which I downloaded on June 7 has its thumbnail while episode 26 which was downloaded June 12 does not. This happens for all videos I've added. I've double checked that the images are not corrupt and that the file names match.
[Edit]
I rolled back to 10.9.1, did a refresh and all the missing thumbnails were picked up immediately