Yeah so it’s important to note that this is an experimental feature that we inherited from Chromium, not one we implemented ourselves. As most flags, it is by nature unreliable and will go through many changes (as the Chromium dev team works on it).
So yes it does not appear to work on YouTube – which does seem quite strange to me, considering it’s Googles own implementation, you’d figure it’d work on their most popular site/product – but it does however work on other sites. For example, I was able to enable the flag and have Spotify appear in PIP mode:
So I’m assuming they’re working out all the issues with this feature right now. At any rate there isn’t a lot we at Brave can/will do about this. I would recommending sharing your feedback with Chrome/Chromium team.