Below some hints:
That would suggest that your router would advertise 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 to stop advertising its own IPv6 or to advertise your Pi-hole host machine's IPv6 as DNS server.
You'd have to consult your router's documentation sources on further details for its IPv6 configuration options.
If your router doesn't support configuring IPv6 DNS, you could consider disabling IPv6 altogether, provide…
DHCP is IPv4 only.
https://www.rfc-editor.org/rfc/rfc8106
If you install below on the Raspi, you can inspect what is advertised on your LAN and from which MAC address the RA is coming from:
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,
…
Example outputs in that same thread.
The official docs have a "Router setup" section that also includes TP-Link: