Short version:
Seeing generic names issued by your ISP is not unusual for public IPv6 addresses.
Click here for the long version
With IPv4, your router is the only device in your network that has a public IPv4 address.
That changes with IPv6:
You router is assigned a public IPv6 prefix by your ISP, which it will distribute in your network in return.
Any IPv6 capable device on your network will construct a set of IPv6 addresses for itself by calculating an interface identifier and combining that with an IPv6 prefix.
This means that any such device on your network can now construct a public IPv6 address for itself by using the public IPv6 prefix. Public IPv6 addresses fall into the 2000::/3 address range.
A client may choose any of its assigned addresses to query Pi-hole, be it IPv4 or IPv6, including its public IPv6 address. If a client uses its public IPv6, Pi-hole will try to associate a name for it. As its a public address, Never forward reverse lookups for private IP ranges won't kick in here, and Pi-hole will forward the reverse lookup to one of its upstream servers.
As your ISP owns the address space it assigned to you, it also supplies DNS services for it. DNS records will be populated with generic names derived from the respective IP addresses, unless your plan buys you DNS services as well (which is unlikely for consumer plans).
Usually, the same would happen for the public IPv4 address of your router - try a reverse lookup for your IPv4.
You may coerce clients into sending queries exclusively via their link-local IPv6 by distributing Pi-hole's link-local IPv6 (from fe80::/10) as DNS. This is somehow dirty, as that address is only visible on the same network segment (or link). So use any L3 switches or VLANs, and devices connected through those won't be able to use Pi-hole that way.
Note that with potentially every device on your network having a public IPv6 address, every device is now potentially reachable from public internet. Whether it is actually exposed in that way will depend on your router's proper firewalling.