The outgoing response traffic from Identity Servers to the IPv6 clients will be first routed to IPv4-Internal and forwarded back to the clients with source IP address as IDP-v6 address.
Because it is Post profile, incoming traffic will be only from IPv6 clients. The clients can be configured with IPv4 address, IPv6 address, or both (dual stack). If the client is configured to use IPv6 address only or dual stack, it must resolve the published DNS name of IDP to IDP-v6 address.