Thanks to the help of the nice folks on the Jellyfin support forum I was able to figure out why my Jellyfin clients weren't fully loading all the media artwork. My hunch was that Nginx was fouling something up. Turns out I was right but it's actually the result of a Jellyfin bug. Here's the forum post for ref: https://forum.jellyfin.org/t-solved-only-blurred-placeholder-images-being-displayed