OK I know this sounds nuts but I did a Debian 10->11 upgrade and then a Pihole upgrade to V6. Later that day I noticed some of the equip (phones and a laptop) would still connect to my main Wifi SSID but report no Internet connection. Guest SSID connections were fine from these devices. I was on the phone with Verizon and they thought they fixed it but it rebroke within minutes.
I later finally gave up on the DNS server failure on PiHole V6 and noticed later my main SSID problems went away after I rolled back to the HyperV checkpoint at Deb 10 / Pihole 5.25.2
Now I'm not network guy but how does this Debian/Pihole upgrades break a specific wifi SSID for SOME devices?
Please take this with a grain of salt since I am making some assumptions about your network.
If you've not set your pi-hole to be IPv6 compatible and your router/firewall is, that would cause your IPv6 clients to not use your pi-hole for DNS resolution. When you performed the upgrade on your pi-hole, it did not quite finish right and caused DNS resolution for your IPv4 clients to stop. This would look like the WiFi is "down". If you use pi-hole for DHCP this could also cause this to occur, as DHCP outages tend to stagger depending on your device lease/renewal times.
Some of your clients are bypassing your pi-hole, since they had connectivity even when your pi-hole was down. I'd suggest looking at your IPv6 settings in your router/firewall, (and even disabling them if you'd prefer) as that is the most common way pi-hole gets bypassed
I'd suggest trying the upgrade again and sending a pi-hole debug log while you are running v6 so the devs can take a look.