Really?, thats unusual because for example I use 6RD on Comcast with a hurricane electric tunnel and it works. Comcast turned off their own 6RD relays back in June.
Not sure how it's working. Maybe you'll want to run »test-ipv6.com/ again.
6RD encodes your public IPv4 address into a /32 subnet on IPv6. That won't work with static 6in4 tunnels as address blocks are already pre-defined for you.