Happens on my shield sometimes too, not quite sure why. I just put it down to my external HDD being mechanical and slow.
Guessing you killed all apps before the reboot too? This has happened to me before but think this fixed it.
Why is your Prime video app so old? Seems like you have bunch of app updates pending. Maybe Plex one as well?
Did you try putting it in rice?
This has been happening to a lot of my app icons. I think my VPN is causing it.
I've not got a VPN, my server is on a Nuc with a NAS storage for files and i've recently experienced this on my Shield too. No idea why and just figured it'd stop annoying me when the show gets bumped off the list. It's only been there a week or two and I haven't seen any others do this but figured it was a Plex issue as the thumbnail is also problematic when I view Plex in a browser window.
shrug
You clear the Plex cache?
Clear your cache
I usually find opening Plex and then closing it helps refresh the icons.
Something to do with Plex right now. It's been happening on my Phone as well as my shield.
Happens to me. You need to load Plex and re-select the server and then the thumbnails re-appear.