This behavior can be fixed by setting
/etc/pihole/pihole-FTL.conf(you need to create this file if it doesn’t exist).
If you still see blacklisted IPv6 domains show as forwarded in your query log but don’t have IPv6 enabled, you may see a false positive like the image below, where the IPv4 version of the domain is blocked, but not the IPv6 version.
root@pihole:~ $ cat /etc/pihole/gravity.list | grep edge.quantserve.com 192.168.1.126 edge.quantserve.com
If your Pi-hole does not have an IPv6 address assigned to it, Pi-hole doesn’t know to use IPv6 blocking and it instead sends the DNS query to an upstream server. If the upstream server returns an AAAA record, but your client devices cannot communicate via IPv6, you will see this behavior.
Fixing these false-positives
If you don’t want to set the FTL config file up the you need to set up a Unique Local Address (ULA) for IPv6 on the network.