It looks like this might be tied to the browser engine... it works for me on Blink (Chrome), but breaks on Safari’s WebKit.

If you're seeing this on iOS, a quick fix is clearing the cache:

Head to Settings > Safari > Advanced > Website Data.

Search for youtube and swipe to delete.

Once you reload the page, the thumbnails should pull cleanly from YouTube's CDN.


PS
Talking about YT thumbnails I confess this one here makes me LOL ..


The coincidence was that I used the "Jenga Tower" analogy to describe when a software breaks .. the inspiration actually came from one of my favorites comic strips XKCD "dependency"
😁


and the whole point to use Eelco Dolstra's nix.