I am using PPPoE and DHCPv6 client to get an /56 (dynamic) from my ISP. The client creates a dynamic pool with prefix length /64.
I have configured an IPv6 address as "::/64", which is automatically filled when I receive a prefix.
I don't use DHCPv6 server, just ND/Interfaces so my devices get an IPv6 address via SLAAC.
I have noticed that if the prefix changes for a reason (uplink disconnection) or if the prefix lifetime is over, my clients don't "forget" their already owned IPv6 addresses. So after some days, I see the following:
This happens to all my devices. I am sure I'm missing something on my IPv6 configuratuion on Routerboard.