After reboot, the DNS resolver must be restarted before it will advertise the ipv6 DNS address of the router.
When pfsense ipv6 is configured with DHCPv6 disabled and RA in "Unmanaged" mode, then after reboot, until the resolver is restarted the DNS does not know that the router has an ipv6 address.
In my configuration I have the DCHP Registration unchecked in the resolver, so I do not have unbound getting restarted due to DHCP events.
After reboot, with a working ipv6 setup, if you go to Diagnostics/DNS lookup and lookup pfsense.mydomain.com, it will only reply with ipv4 address of router.
Examining /var/unbound/host_entries.conf I see that the local-data: lines for the ipv6 for router are missing.
Looking at /var/etc/radvd.conf is missing a lot of information as if it hasn't been configured yet.
Packet capture of RA packets show that indeed, the DNS is not being advertised.
Manually restarting unbound service fixes all of this and it is good until next reboot.
I am running 2.4.4-RELEASE-p3 (amd64)
Installed Packages (latest version):