After installing pihole (but not right after which made troubleshooting challenging, it took about a week to demonstrate the problem) my android device, a pixel 5, began constantly disconnecting from wifi. I finally narrowed it down to pihole on IPv6. Using public dns for IPv6 and pihole for IPv4 everything worked fine and the phone connected. As soon as I set dhcp to use pihole for IPv6, the phone started disonnecting again. I could not find any blocked queries from my phone at all, let alone something that would indicate a connectivity test was being blocked.
My pihole installation is 100% standard with some public adlists installed and a very small collection of blocked domains in a local file, none of which have anything to do with google or android.
I'm stumped and curious if this has been seen before....my keyword fu may not be great because I found nothing when searching the forum.
IPv6's equivalent would be DHCPv6 - different protocol, different ports.
Android exclusively uses SLAAC/NDP for IPv6 - it does not support DHCPv6 at all.
If DNS would be involved, it could only be related to Pi-hole -or one of its upstreams, if you opted for some filtering public DNS service- blocking a domain that your client requires to resolve.
The following topic may help in identifying domains involved: