Reverse resolving IPv6 Privacy Extension addresses to a fixed hostname would defeat their very purpose of obfuscating a client IPs identity.
In addition, your ISP is usually in control of DNS for your public IPv6 addressses, so I'd expect to see generic names in a pattern similar to <somehow-transcoded-IPv6-address>.<ISP-domain-name> for a reverse lookup.
Note that Privacy Extensions themselves are not your issue:
It is the fact that some of your clients may send their DNS request from their IPv6 PE address, and those then would hamper associating a host name.
There is not one fail-safe path to address this, but you may try the following options:
a) Configure your router to not provide an IPv6 DNS server address at all.
In absence of an IPv6 DNS server address, clients will always use their IPv4 for DNS. Hence it's important that your router doesn't offer its own IPv6 nor any other. Depending on your router, that may not be configurable.
b) configure a ULA address for Pi-hole.
This may or may not reduce the amount of queries posed via a public IPv6 address, depending on a client's IPv6 prefix policies (see next)
c) change a client's IPv6 prefix policies to prefer IPv4 (or IPv6 ULA) over public IPv6 in your local network.
This isn't supported on every device (e.g. IoT or smartphones).
d) configure your client to stop creating temporary IPv6 Privacy extension addresses and create a local DNS record for the public IPv6, overlaying your ISP's generic names
As those addresses are meant to improve privacy by regularly changing your IP address (e.g once every two hours), I do not recommend doing so
e) configure your router to make use of your Pi-hole machine's link-local IPv6 address (range fe80::/10 ).
This would result in your clients sending DNS requests from their link-locals.
This is only on an option as long as there are no L3 switching devices in your network, since link-local addresses are accessible by same link or same network segment devices only.
It should be safe if all of your devices are connected directly to your router.