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, provided you'd not depend on IPv6 for reasons.
If your router doesn't support that either, your IPv6-capable clients will always be able to bypass Pi-hole via IPv6.