My ISP is BT in the UK, they support IPv6, here's a picture of the IPv6 settings page on my router. I've tried setting IPv6 allocation to off, which results in nothing getting an IPv6 address.
The Pi is my DHCP server, and IPv6 setting is turned on.
If it is stateless or stateful, devices get IPv6 addresses but it does not seem to be coming from the Pi-hole as they are not listed in it's DHCP leases list.
Using stateless autoconfig does not generate DHCP leases, at least on the two routers I'm familiar with.
If you can force the PI to use DHCP6 it will then have an entry in the leases table that you can use to configure a reservation that will show up in DNS. Be wary of doing that if your ISP uses Prefix Delegation as a change in PD by the ISP can make a mess of any static/reserved stuff you have set up.
