When I'm using my HE /48 and I copy and paste a link into the signal messenger app, I'm unable to render previews. Similarly when I'm on reddit and there is an embedded YouTube link, I am usually unable to play it and have to click around to find the actual URL and paste it into a new browser window. If I disable tunnel broker and use IPv4 or my 6rd prefix, everything works great again. I'm assuming this is something on Google's side and figured there wasn't much we could do about it, but figured I'd ask.
I experience the same so just block AAAA DNS records for YouTube on my Pi-Hole setup with the following regex entries:
(\.|^)youtube\.com$;querytype=AAAA
(\.|^)youtu\.be$;querytype=AAAA
Not the best solution, but it works.