DHCPv6 DDNS doesn't work when the interface is using a PD provided address

RMO

Scenario:
-Have a BIND server set up for dynamic updates using a key
-Said BIND server has the necessary forward and reverse zones set up, and has the ACLs set to allow update using the key
-Configure an interface in pfSense to use DHCPv6 and configure it to do a Dynamic Update toward the BIND server using the key as authentication

If the interface is configured using a static IPv6 range, this configuration is successful.

If the interface is getting it's IPv6 range through IPv6 Prefix Delegation, the DHCPv6 server crashes with the following error:

/etc/dhcpdv6.conf line 70: unknown key ddns-key
key "ddns-key"
^
Configuration file errors encountered -- exiting
If you think you have received this message due to a bug rather
than a configuration issue please read the section on submitting
bugs on either our web page at www.isc.org or in the README file
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exiting.

tested on pfSense Plus 21.09-RC

the same DDNS configuration works perfectly on any IPv4 interface as well, but of course IPv4 interfaces are always statically defined just like in the working IPv6 scenario here.