I've noticed on most of my posts today the thumbnail is not appearing on lemm.ee - for example I've just posted https://lemm.ee/post/1477918 and there is no thumbnail but if I view the post on lemmy.world - https://lemmy.world/post/1733425 - there is a thumbnail so it only seems to affect this server.

The other posts I've made today the thumbnail appeared after I edited the post a few hours later (making no changes just clicking edit then save) but the post above this doesn't seem to work for.

This isn't specific to youtube links as one of the posts I made today was from the BBC.