Configure ipv6 gateway in VETH, can ping the gateway address in the container, but the next hop can't forward the data, meaning that the ipv6 gateway doesn't work
PS。GW is the new private address GW set up on the mikrotik via NAT66
As of 7.12.1 I have IPv6 in containers working stable. Tested both from inside (/container shell 0, then ping 2001:4860:4860::8888, for example) and outside (ping container address from a third machine connected to the host).
Look at your filter/nat firewall config and test it on real machines before containers.