Enable IPv6 privacy in NetworkManager
On most IPv6-enabled networks, network addresses are distributed via stateless address autoconfiguration (SLAAC). That is a fancy way to say that hosts on an IPv6 network will configure their own IP addresses.
The process usually works like this:
The host sends out a router solicitation request: Hey, who is the router around here? The router replies with a prefix: I am the router and your IPv6 address should start with this prefix.