Why would you make the internal network dual stack? Now you need to two sets of firewall rules, and devices have two protocols to reach each other making debugging unnecessarily difficult.

Most traditional internal networks are IPv4 only. In larger tech companies where 10.0.0.0/8 isn't enough, internal networks are IPv6 only. Only a tiny number of Internet-facing servers (like your load balancer / reverse proxy for external traffic) should be dual-stack.