On the M5 I have configured IPv6 in 'Bridge' mode so that the mesh passes IPv6 traffic at layer 2. The ISP CPE router (TP-link Archer C80) is using DHCPv6.

When a host connects, it receives an IPv6 address and gateway but no IPv6 DNS addresses.The host can ping the IPV6 WAN IP of the CPE. If I connect the same host directly to the CPE it does receive the IPv6 DNS addresses. I cannot see how the CPE would be aware of any difference between a host connecting through the M5 or connecting directly so it appears the M5 is somehow blocking the DNS addresses from being passed through to the hosts.