I’d be surprised if that test worked on any platform in existence. You’re using the markdown to render a static image, and sticking a youtube URL in there. PieFed supports it if the URL ends in something like .mp4, but that’s only because Lemmy have fudged it, and so now people expect it. There’s meant to be a 1:1 relationship between markdown and HTML, metadata transformed into metadata - nothing should have to look at the actual contents to know what tags to produce.

As for ‘works for me embedded in mbin’ … eh? It looks like this in mbin:

That’s literally just a external link to youtube. It ‘works’ because it doesn’t - same as for the screenshot itself - instead of embedding it, it just coughs up the link to a remote site. Everything else is rendering it as it is - a broken link to an image that doesn’t exist.

(maybe ‘originallucifer’ has some fancy app that takes a youtube shorts URL, works out the embed code, and then puts it in an iframe … but like I say, I’d be surprised).