Mountain View, CA
That means you're at least picking up the /64 prefix delegation that Comcast hands you (which is the IPv6 network block that should be used by systems on your LAN for direct IPv6 connectivity), so that means at least some RAs are working.
I have no explanation for why there's no default IPv6 gateway other than a network misconfiguration on Comcast's side or a firmware/router bug in the E3200. An alternate possibility is that Cisco DPC3000 is incorrectly filtering or messing with some kinds of IPv6 packets (strange but possible), even though cable modems aren't supposed to screw with layer 3 traffic passed through to the underlying LAN port. That's about all I can do with the information here -- IP network technicians have to figure out the rest. I'd wait for NetDog at this point.
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.