I have a home network with a router provided by the ISP (Free in France). This router supports IPv6 and broadcasts RA which includes the DNS to use. It also offers DHCP services and forwards DNS queries.
To simplify things, let's assume I started with this setup - everything works fine, and all my Wi-Fi devices connect correctly.
I then decided to add a host with Pihole. The idea is to replace with it the router's DHCP and DNS functionality and disable the DHCP server on the router). I installed the service and started to distribute via its DHCP the IPv4 DNS address (= the address of the Pihole).
It worked everywhere, except for a single Pixel 6 phone that started to continuously disconnect and reconnect to the WiFi, in a loop (as soon as it would connect, it would disconnect - the frequency was about a second).
I spend countless hours trying to understand what that phone fails (out of many other Wifi devices that work great: phones, radios, Chromecasts, Google minis, my own Arduino IoTs, Tuya devices, ESPHome devices, ... → all of them worked fine).
Because of unrelated issues I realized that the ISP router is sending a faulty IPv6 DNS server address as part of its RA (it was sending a non-existing address). I fixed that and immediately the Pixel 6 started to behave normally (connects to the WiFi and stays there).
My question: how can a faulty IPv6 DNS advertisement (part of the RA) impact the WiFi connection (specifically: the continuous presence in that WiFi, the low-level connexion went fine) of a Pixel 6?
My question is more general, but I only witnessed first-hand the issue with a Pixel 6.