We are experiencing similar behavior, where 6rd traffic leaves the Sonic.net network, but fails when it comes to end-to-end connectivity. We are actively working to fix these issues, and I will post an update once I have some news. At this exact moment, we have our 6rd prefix configured for 2602:240::/32 and it is working fine for me. The disadvantage, is that this only allows customers a single /64 network for use. If anyone would like to try testing their configuration with a /32 prefix, I would love to know whether it is working for you or not.
Sounds like the /28, only the /32, is getting advertised. It is
possible to give shorter IPv6 prefixes if you can omit the IPv4 prefixes (sinec those parts tend to be redundant), but ideally that would be combined with using an IPv6 /28 to give a /56 to each user as recommended by RfC. But a /60 would be fine too.