Custom DDNS updates for IPv6 ... don't update

[inthewilds]

Are there known issues with 'Custom (v6)' IPv6 DDNS automatic update not updating at all?

I'm trying to use 'Custom (v6)' DDNS updates to pass the OPNSense router's IPv6 address to a custom API endpoint, but it's not working. The general log shows "/services_dyndns_edit.php: Dynamic DNS: (Success) IP Address Updated Successfully!". However,  the API endpoint is never contacted and tcpdump (on the OPNSense box) shows that OPNSense isn't making an outbound connection attempt.

Custom DDNS updates for IPv4 are working fine with the same endpoint.