pfSense

I also found that 'AdvRouterAddr on' is used everywhere in radvd.conf, but this is incorrect (radvd.conf(5)):
When set, indicates that the address of interface is sent instead of network prefix, as is required by Mobile IPv6. When set, minimum limits specified by Mobile IPv6 are used for MinRtrAdvInterval and MaxRtrAdvInterval.

or is it some kind of workaround?

fix:
https://github.com/pfsense/pfsense/pull/4199

Feedback: This has been implemented incorrectly.

Tested on:
2.5.0-DEVELOPMENT (amd64)
built on Thu Sep 10 13:02:00 EDT 2020
FreeBSD 12.2-PRERELEASE

This change in https://github.com/pfsense/pfsense/pull/4199 sets the 'AdvRouterAddr' on for unmanaged networks.

The 'AdvRouterAddr' flag should only be set if the router implements ipv6 mobility. As far as I can tell pfSense does not implement ipv6 mobility or if it does it provides no facility to configure it. If it did provide this facility it is not related to whether it is "Unmanaged" or any of the other radvd settings.

Correct implementation would have been to remove the 'AdvRouterAddr' and set the prefix to all zeros in the right portion.
i.e.
This is what is generated for the prefix:

prefix fd04:6ddc:fe8e:e010::1:1/64 {
        DeprecatePrefix on;
        AdvOnLink on;
        AdvAutonomous on;
        AdvRouterAddr on;
        AdvValidLifetime 86400;
        AdvPreferredLifetime 14400;
    };

This is what should have been generated:
prefix fd04:6ddc:fe8e:e010::/64 {
DeprecatePrefix on;
AdvOnLink on;
AdvAutonomous on;
AdvValidLifetime 86400;
AdvPreferredLifetime 14400;
};

I think this would be dependent on Feature 6827, Add Proxy Mobile IPv6 (PMIPv6)

Right, https://tools.ietf.org/html/rfc6275#page-65:

  Mobile IPv6 extends Neighbor Discovery to allow a router to advertise
  its global address, by the addition of a single flag bit in the
  format of a Prefix Information option for use in Router Advertisement
  messages.

Fix:
https://github.com/pfsense/pfsense/pull/4449

Feedback:
Tested on:
2.5.0-DEVELOPMENT (amd64)
built on Mon Sep 21 07:00:38 EDT 2020
FreeBSD 12.2-PRERELEASE

Results of radvd.conf are as expected now.

# Automatically Generated, do not edit
# Generated for DHCPv6 Server lan
interface hn1 {
    AdvSendAdvert on;
    MinRtrAdvInterval 5;
    MaxRtrAdvInterval 20;
    AdvLinkMTU 1500;
    AdvDefaultPreference medium;
    prefix fd04:6ddc:fe8e:e010::/64 {
        DeprecatePrefix on;
        AdvOnLink on;
        AdvAutonomous on;
        AdvValidLifetime 86400;
        AdvPreferredLifetime 14400;
    };
    route ::/0 {
        AdvRoutePreference medium;
        RemoveRoute on;
    };
    RDNSS fd04:6ddc:fe8e:e010::1:1 { };
    DNSSL localdomain  { };
};