Ok.
I am curious now, Can you please explain how are you enabling your core to route VPNv6 prefix? by enabling family inet6-vpn under protocol bgp?
do you see hidden routes in vrf/routing-instance table?
For IPv6 route, you need to have IPv6 next-hop. I am not sure how you are doing this. try enabling ipv6-tunneling inder protocol mpls and check.
Regards