Missing YouTube video thumbnails [fixed] - eviltoast

Does anybody know why and when thumbnails for YouTube videos are shown or not? It appears to depend on the lemmy instance the posts are served from:

Without Thumbnails

With Thumbnails

Update: It’s not only YT thumbnails which aren’t shown. This is your post about the free 24 slot bag as shown on lemmy.wtf (without thumbnail), lemmy.world (without thumbnail) and feddit.de (with thumbnail):