pfSense

Suggesting the following change (based on the services.inc in 2.4.5):

--- old/src/etc/inc/services.inc 2020-03-17 18:55:25.044950000 0100
++ new/src/etc/inc/services.inc 2020-03-17 18:54:24.178128000 +0100
@ -1610,6 +1610,7 @

if ($nsupdate) {
         $dhcpdv6conf .= "ddns-update-style interim;\n";
+         $dhcpdv6conf .= "update-static-leases on;\n";
     } else {
         $dhcpdv6conf .= "ddns-update-style none;\n";
     }

Right, https://ftp.isc.org/isc/dhcp/4.4.1/dhcp-4.4.1-RELNOTES:

- The server now honors update-static-leases parameter for static DHCPv6
  hosts.

PR:
https://github.com/pfsense/pfsense/pull/4263

Side note: I'm using this modified configuration on my 2.4.5 as a patch since 4/02/2020 and updates my bind9 with all the reserved IPv6 addresses perfectly.