Can't speak for OP, but there's an annoying thing going on with links in that it doesn't always actually bring in a thumbnail when you do a link. So you think--oh, I'll just add my own thumbnail too! Then I'll have a link to the video with my custom thumbnail! But what that does is remove the video link while preserving the thumbnail.

It's one of those UI things that I guess hasn't been worked out. Or maybe it's something under the UI with Lemmy itself.

Either way, you get posts like this.