Ipv6 transformed into readable hostname only after restart of DNS resolver

With IPv4, a DHCP client will indicate its hostname when registering for a lease, allowing a DHCP server to inject that name into a co-hosted DNS server (not all routers will actually do this - your FritzBox does).

There is no equivalent mechanism for IPv6.

This turns reverse lookups for local IPv6 addresses into a guessing game of sorts.
pihole-FTL/dnsmasq would apply some heuristics to associate a given dual-stack machine's IPv6 adress with the hostname for an IPv4 of the same host.
It may be possible that your FB does something similar, but in either case, that's not guaranteed to always work.

On top of that, IPv6 addresses are designed to change regularly, and public GUA IPv6 addresses are resolved by your ISP's public DNS servers, so the hostname will be administrated by them (you'll often see a generic name for those that can be mistaken for an IPv6 on first glance, e.g. somehing similar to 2a02-abcd-dead-f00d-1234-5678-abcd-ef00.pool.telco.com).

To avoid this, I'd recommend to NOT advertise or offer an IPv6 DNS resolver address in your home network (again, not all routers would support this - your FB does). That way, clients will talk to your Pi-hole strictly via IPv4, making it a lot easier to associate them with hostnames (see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn).
As DNS is indifferent to the transport protocol used, your Pi-hole will continue to supply AAAA as well as A replies to your clients as requested, allowing them -as ever- to use IPv6 addresses from those AAAA records or IPv4s from A to establish communications as they prefer.