OS is Debian stretch. After years of running our firewall on Debian without issue we've decided to start allowing IPV6 connections as well. Our public interface is eth0. If possible we would like it to configure itself.

/etc/network/interfaces contains the following lines, plus a bunch of internal VLANs on eth1 which aren't relevant to this post:
allow-hotplug eth0
iface eth0 inet dhcp
iface eth0 inet6 auto

eth0 configures its link-local address fine but doesn't seem to do anything with the router advertisement. We still don't have a global unicast address.

Yes net.ipv6.conf.eth0.accept_ra = 2
Yes nftables is allowing router advertisements to eth0
Yes we've looked at the firewall logs in /var/log/messages and the router advertisements aren't being blocked.
And yes tcpdump shows the router advertisements being received by eth0.
We've also allowed eth0 to output router solicitations.
Neighbor advertisements/solicitations are allowed in both directions.

I actually wouldn't mind configuring eth0 statically if I knew what my assigned network/prefix was. You would think logging in to Comcast's website would give you this information, but that would make too much sense.

Any new ideas?