There is a list of disallowed HTML tags in the Joplin source code. It will strip them out if you try to use them (see https://discourse.joplinapp.org/t/how-to-use-iframe-tag-and-render-in-joplin/32421). I assume that this is happening here as well. I'm actually very surprised that you've got the embedded video to display at all, as in my testing, this only worked after modifying the source code to allow the blocked tags.