The ISP is Spectrum/TimeWarner. I'll call them tomorrow, but you don't get to talk to engineers, just call center people. Most of them can't even spell IPv6, much less tell you anything about it.
Another odd thing, I rebooted pfSense and then checked the DHCP logs to see if I could confirm the /56 that was being handed out. I didn't see anything in the log at all for dhcp6 (yes, there was stuff in there before...) :
# clog /var/log/dhcpd.log | grep dhcp6
# <------ ¯\_(ツ)_/¯
radvdump shows that the /64s being offered there have valid lifetimes of infinity :
prefix 2604:2000:ffc0:4::/64
{
AdvValidLifetime infinity; # (0xffffffff)
AdvPreferredLifetime infinity; # (0xffffffff)
AdvOnLink on;
AdvAutonomous off;
AdvRouterAddr off;
}; # End of prefix definition
So maybe pfSense is caching this and just re-using it without triggering dhcp6c at all? I know it's not in config.xml so maybe on the filesystem somewhere? I have to look at the code to see if this is what's actually happening.
I'll try to do some more packet captures as well.