Outbound IPv6 traceroutes end after 2602:24b:8179:10::1 (cust-gw.ipv6.sonic.net), so I think this is an issue with Sonic, since I can get IPv6 traffic to their gateway. An inbound traceroute from Hurricane Electric ends at 10ge1-18.core1.sea1.he.net, so I'm guessing it doesn't have a route announced. I also noticed that the BGP route announcement for the Sonic 6rd block (2602:240::/28) is marked as having problems.
I note that the IRR origin for 2602:240::/28 is AS7065 and not AS46375, which is where the announcement is coming from, but now I'm over my head in terms of what I know about BGP.
My 6rd endpoint is a pfSense-based router, not the Sonic modem. I've got static IPv4s. I've been using this configuration for quite some time.