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):
The docker-compose.yml I copied from somewhere when setting up lemmy.world apparently was missing the external network for the pictrs container.. So pictrs was working, as long as it got the images via Lemmy.
Getting the images via URL didn't work...
Note: This only fixes new posts. In order to fix old posts you need to edit/save them once.
I've been trying to troubleshoot it. It makes very little sense to me, that it doesn't show thumbnails from local posts on lemmy.wtf, but if you can via another instance, the thumbnails show up.