Do you have working IPv6 upstream connectivity into the Internet?
Use, e.g., https://test-ipv6.com/ (I get a 10/10 myself) or https://ipv6-test.com/ (I get a 17/20 over there). If you do not have IPv6 connectivity this test is expected to fail. The first test only succeeded because this was trying if your Pi-hole itself is reachable locally over IPv6 - which it always is unless you have disabled IPv6 system-wide by some custom configuration.