You are observing the expected behaviour for reverse-resolution of a public client IPv6 address.
Your ISP owns the IPv6 address space it has assigned to you and is thus handling DNS for it as well.
As it is a client's decision how to pick the IP address it uses to issue DNS requests to Pi-hole, there is nothing you can configure on Pi-hole's side (at least as long as IPv6 Privacy Extensions are used by your clients).
Instead, you'd have to configure each of your clients and/or your router to change that (see e.g. Using pihole as DHCP server but ipv6 address showing in logs - #6 by Bucking_Horn for a non-comprehensive choice of options).