Your Pi-hole v5 debug log suggests that clients would be aware of a few IPv6 addresses for DNS:
-rw-r--r-- 1 root root 166 Mar 3 04:09 /etc/resolv.conf
search fritz.box
nameserver 192.168.178.1
nameserver fd<redacted>e7
nameserver 20<redcated>
Those IPv6 addresses belong to your Fritzbox router, allowing clients to by-pass Pi-hole via IPv6 at their own discretion.
Fortunately, Fritzbox routers allow to disable propagation of IPv6 DNS servers - see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.