I have exactly the same problem: a topic with 1500+ posts many of those with youtube video links. And ones at the start of the topic do not render any embed, while those which are recent do render embeds.
All youtube links which do not render are actually valid and if you open them in the separate tab, they play on youtube perfectly. Or if you create a new post with the same link, it will render an embed.
Seems like a bug in Discourse?
Version: 3.1.2