Ok, you said not to put a static IPv6 but there is a flow.
I had to update my nas and so I had to restart the vm with pihole.
The Ipv6 of pihole changed but Pihole doesn't see the change.
[✓] IPv6 address(es) bound to the enp0s4 interface:
fdb4:2e:bf52:56d8:7104:e7e6:92e0:8bce does not match the IP found in /etc/pihole/setupVars.conf (Use IPv6 ULA addresses for Pi-hole)
2a02:a03f:a59d:5301:fb0e:a726:67f9:d49e does not match the IP found in /etc/pihole/setupVars.conf (Use IPv6 ULA addresses for Pi-hole)
fdb4:2e:bf52:56d8:2a0:98ff:fe25:1d67 does not match the IP found in /etc/pihole/setupVars.conf (Use IPv6 ULA addresses for Pi-hole)
2a02:a03f:a59d:5301:2a0:98ff:fe25:1d67 does not match the IP found in /etc/pihole/setupVars.conf (Use IPv6 ULA addresses for Pi-hole)
fe80::2a0:98ff:fe25:1d67 does not match the IP found in /etc/pihole/setupVars.conf (Use IPv6 ULA addresses for Pi-hole)
It's not pratical. It makes more sense to have a static ip.
So it stays the same in the fritz.box and /etc/pihole/setupVars.conf
Beside the fritzbox has a security on the dns. It needs a "physical" confirmation. I can't do it every time the ip changed.
The computers see the new IPv6 as DNS.