I don’t believe this is a bug, previously when previewing videos and other media types in the composer they’d re-render on every keystroke… so to avoid that we now use a generic placeholder instead.
https://github.com/discourse/onebox/commit/b987f97e36843365737d9fa944ef209c607044c4
I can see how the placeholders could be interpreted as a broken embed… I wonder if there’s something we can do to clarify…