@nbianca apologies, this might be a combination of two issues
Works on and prior to this with TLP plugin:
https://github.com/discourse/discourse/commit/922c40f87cdceb8197dd61361334e0def23f13d5
After this commit, it fails with TLP.
I agree with you that for YouTube, out of the box that property is not populated.
So it seems like TLP was doing a better job with its overrides, but relying on something which is now changed.
FYI the relevant overrides are here: https://github.com/angusmcleod/discourse-topic-previews/blob/master/lib/cooked_post_processor_edits.rb
I think I will need to do some more analysis, but it looks like there’s been enough of a change to break TLP.
Let me dig into this a little more and revert again. Unfortunately I may not get chance over next couple of days.