IPv6 RA: prefix option does not contain router address in spite of "R" flag being set
When setting router mode to "unmanaged" and not specifying any prefix, pfSense does not send the advertising interface's address in the RA's prefix option even though the "R" flag is being set. Instead it only announces the prefix, with all host bits set to zero. This is in violation of RFC 6275, which states on page 65 regarding the "R" flag: "When set, indicates that the Prefix field contains a complete IP address assigned to the sending router."
The cause of the issue seems to be /var/etc/radvd.conf, which contains the improper configuration of said RA messages.