Try run below one on the Pi:
sudo pihole-FTL dhcp-discover
Find the lines/fields starting with "dns-server" for IPv4,
or "Recursive DNS server" for IPv6 ... if any is/are listed
The "problem" is that if any of those addresses are not those of the Pi host, clients would be able to bypass Pi-hole via this IPv6 DNS IP and ads etc will leak in:
Also most implementations prefer IPv6 over IPv4 DNS.
If you run below on the Pi, it wil list IPv4 addresses configured on all detected interfaces for you to compare with the dhcp-discover output:
ip -br -4 address
And below will list IPv6 addresses:
ip -br -6 address
Dont post any of the output for above commands here for privacy!
Also have a look at the docs for more options/details etc: