I can’t understand your goal. DNS servers give you a (usually static) IP address for a domain name, depending on the request IPv4 and/or IPv6.
Dynv6 has the purpose of dynamically adapting these IP addresses to the changing address of your router. The IPv4 address can only point to your router (which can forward the requests via port forwarding to a computer in your network behind the router), the IPv6 address points to the IPv6 network (the first 64 bits) assigned to your router by your ISP. In other words, the second 64 bits decide to which computer (or the router) in your IPv6 network the packets are sent.
It therefore makes no sense to change the IPv6 prefix only for certain addresses, as these can then no longer be reached (in your IPv6 network). The packets are then sent somewhere, whether the network can be reached there (someone will definitely be happy about these packets, who knows which - perhaps bad - packets he is sending back) or not.
If you do not want the packets to be answered, you should change the host part (the second 64 bits) so that they point to an address that does not exist in your network. As I said, what sense that should make is not apparent to me. If you explain this in more detail, it may be possible to find a different/better solution.