you can check Unbound logs
journalctl -u unbound
As well, did you tried to configure PiHole to use IPv4 to connect to Unbound? (custom#1)
And just for testing. How does it behave if you select one of the IPv4 options for a global upstream DNS provider instead of Unbound? Again, for PiHole itself, it is not important to use IPv6 to connect to an upstream DNS server. IPv4 is totally fine. It’s just the user client that would need to have to IPv6 connection to PiHole.