Try restarting Tor Browser and check headers the first time you access bitcointalk.org (make sure to have the dev tools open before you access the site). I think the header is sent only when the browser connects via an exit node, which would be the first connection, and after that the browser may already be connecting via onion so would not be getting the alt-svc header.
Also check if you have alt-svc enabled:
Now i can see the alt-svc parameter on response header. But the weird thing i can't use the onion link to access this forum, with or without https:// prefix.