I had this same problem when I tried to use a Gutenburg youtube embed block on a PAGE (not a post). Video showed fine in editor but in preview or upon publishing, the video would be replaced with the youtube url (which was not a link, just a url). After A LOT of trying different things I inadvertently discovered the cause of the problem (at least in my case). I had the embedded youtube block part of a reusable block and that caused the problem. As soon as I converted the reusable block to a regular block, the video showed up fine. Hope this helps someone else.
