<feed ... xmlns:media="http://search.yahoo.com/mrss/">
It appears that the YouTube’s feed uses the Media RSS extension. Both Discourse core and the Wellfed plugin relies on the Ruby RSS module to parse feeds, but it does not support that extension.
I don’t have plans to add supports for the extension, but I am open to PRs! I imagine the change will need to teach the Ruby RSS module and DiscourseWellfed::FeedItem about the new elements.
Could you share (or DM me):
- the feed URL, and
- your set up for that feed in
Customize > Embedding > Allowed Hosts