pfSense

According to 2.3.3 New Features and Changes it is now possible to force a Dynamic DNS hostname in DHCP/DHCP6 Server settings. All is working as intended for the DHCPv4 server side, checking DDNS Hostnames generates a ddns-hostname "randomname" definition in the lease section of the dhcpd.conf.
host s_lan_7 {
hardware ethernet 58:48:22:60:c8:7d;
option dhcp-client-identifier "00:01:00:01:1f:e1:db:25:52:98:93:c3:f2:f0";
fixed-address 10.0.200.11;
option host-name "randomname";
ddns-hostname "randomname";
Also in the Static DHCP Mapping on LAN you have the option to check DDNS Hostname.

Now on the DHCPv6 server side of things after checking the DDNS Hostnames checkbox the relevant section in dhcpdv6.conf doesn't contain any reference whatsoever to a ddns-hostname option :
host s_lan_4 {
host-identifier option dhcp6.client-id 00:01:00:01:1f:e1:db:25:52:98:93:c3:f2:f0;
fixed-address6 2a02:beef:cafe:0001::d;
option host-name randomname;
}
There is no mention whatsoever of a DDNS Hostname option in the Static DHCPv6 Mapping.