Are the queries going over IPv6, or are they requesting an IPv6 response (the latter is true if you read IPv6 in the Query Log).
If the latter case is true:
You don't need an upstream IPv6 DNS server, but if your network has IPv6 internet connectivity (http://ipv6-test.com/) then you should choose both IPv4 and IPv6 in pihole -r. However, if you don't have IPv6 connectivity, then the IPv6 queries are basically worthless because the client can't use the response, so you can ignore those queries via: