You setup your network to distribute your router as DNS server via DHCP. This is a valid setup. However, did you set Pi-hole to be the only upstream DNS server in the router?
*** [ DIAGNOSING ]: Networking
[✓] IPv4 address(es) bound to the eth0 interface:
192.168.178.49/24
* Received 548 bytes from eth0:192.168.178.1
Offered IP address: 192.168.178.71
Server IP address: 192.168.178.1
Relay-agent IP address: N/A
BOOTP server: (empty)
BOOTP file: (empty)
DHCP options:
Message type: DHCPOFFER (2)
server-identifier: 192.168.178.1
lease-time: 864000 ( 10d )
renewal-time: 432000 ( 5d )
rebinding-time: 756000 ( 8d 18h )
netmask: 255.255.255.0
router: 192.168.178.1
dns-server: 192.168.178.1
domain-name: "fritz.box"
broadcast: 192.168.178.255
ntp-server: 192.168.178.1
Port Control Protocol (PCP) server: 192.168.178.1
See here: Fritz!Box (EN) - Pi-hole documentation
P.S. I see you use IPv6 in your local network. But your Pi-hole got an GUA address which is subject to change (e.g. different prefix from ISP). You should assign a ULA to all devices in addition.