That is an invalid assumption.
First, DHCP is strictly IPv4. DHCPv6 is a different thing altogether (different protocol, different ports).
If you didn't enable IPv6 support via Pi-hole's UI, Pi-hole wouldn't react to a client's DHCPv6 requests at all. Even if IPv6 support is enabled, a client has a range of identifcation options to request a DHPCv6 lease, including but not limited to MAC address (actually, the same is true vor DHCP and IPv4 as well, if less common).
Second, in addition to Stateful DHPCv6 (which is similar to IPv4's DHCP) an IPv6 client may also join a network by SLAAC/NDP or Stateless DHCPv6 at its own discretion. Most modern OSs will prefer SLAAC, and there are also OSs that do not support Stateful DHCPv6 at all (e.g. Androids).
You are likely observing correct behaviour.
As far as can be assumed from your screenshots, you are seeing the correct generic public hostnames for your devices' public IPv6 addresses rather than reverse IPv6 addresses.
Your ISP is operating the authoritative DNS servers for your public IPv6 addresses (range
2000::/3), and may well use such an address itself to construct a generic name.