PiHole as a Upstream DNS

[Bucking_Horn]

@Protheus

Now in Query Log i see only the last hop of the DNS Query (Client column) but not the real origin (User Client IP)

This would be the expected behaviour for your setup.

If your Windows DC and BIND servers are ECS-enabled, Support for add-subnet option from dnsmasq (ECS/EDNS0 Client Subnet) may be relevant for you.