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:
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: