if you follow exactly the setup procedures, it was working fine about 1-2 years back on version 6.2 or something, but with the latest release of the RouterOS, I think it break some part of the IPv6 configuration.

to make it work again, 1st, check under your IPV6->DHCPv6 Client, make sure you get an IPv6 prefix from your UNIFI connection.

Then, look under your IPv6->Address and the Interface (or the bridge) that is giving out IPv6 address to your LAN PC or devices connected to it, it must had a Prefix associated with the IP Pool (in this case the prefix would be ::/64 and in red if you follow the setup guide)

if it doesnt or the entry was in red, create a new IPv6 pool then copy the Prefix from the DHCPv6 client that UNIFI assigned to you into that pool. Then change the IP Pool setting in the DHCPv6 client and the pool that is assigned to your bridge or LAN interface to the newly created pool.

Reconnect your LAN interface on your PC, Voila, it is work now.

What actually happened was in some earlier version of Router OS, the LAN Interface or Bridge that was assigned with the IPV6 pool that with prefix of ::/64, and which at the same time this pool is attached the the DHCPv6 client which is getting the prefix from the UNIFI connection, the prefix gets automatically set into the pool. But somehow it doesnt work any more in the latest routerOS, so you had to manually create a new IPv6 pool and set the prefix into it, and change the IPv6 pool from the old one to the new one.

Warning, you do not need to renew the DHCPv6 client connection for your unifi connection else you will get a new prefix then you will need to repeat the whole process by putting the prefix back to the old IP pool and switch it back.

ok someone maybe can test this out in more detail and cook up a script to handle that more effectively.