IPv6 not working in Repeater Mode on 2nd Asus Router

[newsera]

I have an Asus AC-68U acting as the Primary Router running 384.14. When I enable IPv6 and set it to Native Mode, i can see IPv6 address on my interfaces and I can access IPv6 resources. But surprisingly, clients connected to another AC-68U which is on Repeater Mode cannot access IPv6 resources. Simple enough, when I turn off the second router, the same clients are able to access IPv6 resources on the Internet without any issues. So I believe IPv6 connectivity is somewhat broken in Repeater mode.