There's not any common reason I can think of to go ipv6 on the backend. KISS --if there's enough IPs in a private v4 range, use them.
If you need/want to serve v6 clients, put a v6 load balancer in front. You're likely going to need that lb anyway.
If you need/want to serve v6 clients, put a v6 load balancer in front. You're likely going to need that lb anyway.
ipv6 and routable ips is way simpler than patching layer 2 networks together.