this is technically maybe not a bug, but in practice it comes to the same thing.
DHCP within networkd is untweakable.
If you happen to have only IPv6 /64 span that you have to work with within several sub-networks ( like fresh IPv6 user that got his /64 span from ISP and wants to use it) there is no standard way for DHCPv6 server to send a client his subnet ( /64 is assumed ), so one needs to send it through extensions. And catch it on the client side with a hook script.
This functionality doesn't seem to be offered here. I don't need a bash script to be called, but to have a convenient point where an extra C function could be dlopened externally and called, would be very nice.