I just noticed that the topic at OpenDNS on their IPv6 servers had been updated this week. If you are using either set of their servers you might want to check that your pi-hole settings match these addresses.
We support recursive IPv6 DNS resolution and security filtering for IPv6 traffic. Our IPv6 DNS server addresses are:
2620:119:35::35
2620:119:53::53
For users looking for an RFC-compliant DNS service that does not provide any level of filtering, the following IPv6 DNS server addresses can be used instead:
Yes. that second set are but I believe it was the first set that was updated, the filtered ones. At least the v6 IPs are different than I had in my pi-hole settings.
Been using Open DNS for years now and I've been happy with their IPv4 filtered service so I thought I'd keep using the filtered version for v6 too. The quick updates there for malware blocking were one big seller for me, the easy to use blocking choices not so much for me but it has helped friends that want a point and click solution.
If users have selected the filtered DNS it would be in the Web GUI custom settings or in /etc/pihole/setupVars.conf (which allows more than two custom DNS entries).