IPv6 capable clients most often would prefer to communicate with IPv6 targets (unless you'd adjuste a clients IPv6 prefix policies individually).
You should note that DHCP is strictly IPv4.
Stateful DHCPv6 would be roughly equivalent, but most OSs would use SLAAC to join an IPv6 network, where it's your router's job to advertise an IPv6 address as DNS server.
Obviously, your router is advertising its own IPv6 address as DNS server, allowing your clients to by-pass Pi-hole.
You'd have to find a way to configure your router either to advertise your Pi-hole host machine's IPv6 as DNS server or to stop advertising its own.
I'd recommend the latter, and that is supported by FritzBox routers, see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.
In case you did really configure your router to use .local as your local domain name (aka search suffix): *.local FQDNs are reserved for usage by the mDNS protocol as implemented e.g. by Apple's Bonjour and should NOT be used with plain DNS.