[1:root@c3po dhcp]$ rpm -qa | grep dhcp | sort
dhcp-client-4.3.6-8.fc27.x86_64
dhcp-common-4.3.6-8.fc27.noarch
dhcp-libs-4.3.6-8.fc27.x86_64
dhcp-server-4.3.6-8.fc27.x86_64

config:
subnet6 2001:xxxx:yyyy:40::/64 {
...
#    option dhcp6.no-client-update    true;    # unknown
#    option dhcp6.server-update       true;    # unknown
  option dhcp6.fqdn  = concat(config-option server.ddns-hostname, ".", config-option server.ddns-domainname);


When I set the dhcp6.fqdn in DHCPv6 I get:
Jan 14 22:36:18 c3po dhcpd[22792]: Error evaluating option 39
Jan 14 22:36:18 c3po dhcpd[22792]: Sending Reply to fe80::909b:fee1:ea62:2439 port 546
Jan 14 22:36:18 c3po dhcpd[22792]: Added new forward map from Example-PC.lan.example.com to 2001:xxxx:yyyy:40:50f7::37

server.ddns-hostname and server.ddns-domainname are correct and the
DNS is being updated.

Also, you can't use any of the sub-options of dhcp6.fqdn like you can with DHCPv4:
   option fqdn.no-client-update    true;
   option fqdn.server-update       true;
The dhcp6 equivalents show up as 'unknown'.

I've even tried setting the sub-options for fqdn and then using:
     option dhcp6.fqdn        = config-option fqdn.fqdn;
which still generates the 'Error evaluating option 39'.

Please fix.
Bill