You can check what DNS server(s) is/are advertised via router IPv6 RA (Router Advertisement) if install below on the Pi-hole or other Linux host:
sudo apt install ndisc6
FYI:
$ apt show ndisc6
[..]
Description: IPv6 diagnostic tools
ndisc6 gathers a few diagnostic tools for IPv6 networks including:
- ndisc6, which performs ICMPv6 Neighbor Discovery in userland,
- rdisc6, which performs ICMPv6 Router Discovery in userland,
- rltraceroute6, a UDP/ICMP IPv6 implementation of traceroute,
- tcptraceroute6, a TCP/IPv6-based traceroute implementation,
- tcpspray6, a TCP/IP Discard/Echo bandwidth meter,
- addrinfo, easy script interface for hostname and address resolution,
- dnssort, DNS sorting script.
If you run below one, it will show you the MAC address from which the RA is coming from, the IPv6 prefixes and DNS server(s) advertised:
rdisc6 <NETWORK_INTERFACE_NAME>
Maybe it needs sudo in front of it or root powers for your distro.