That would indicate that your router is advertising 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 as DNS server, or to advertise your Pi-hole host machine's IPv6.
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.
You could then try to mitigate this, by setting Pi-hole as the only upstream of your router, provided your router supports it.
But note that you won't be able to attribute DNS requests to original individual IPv6 clients in such a configuration.