I only have 2 x DNS servers configured, and they are my Pi-Holes. Example of one client, which is a Windows 11 PC that has an explicit Group on the Pi-Holes to block 10 million sites (porn, gambling, high risk sites, ad sites, etc) has the following output for google.com vs redtube.com (google is permitted by the configured block lists, redtube.com i blocked):
C:>nslookup google.com 192.168.5.3
Server: UnKnown
Address: 192.168.5.3Non-authoritative answer:
Name: google.com
Addresses: 2404:6800:4006:811::200e
172.217.24.46C:>nslookup redtube.com 192.168.5.3
Server: UnKnown
Address: 192.168.5.3Non-authoritative answer:
Name: redtube.com
Address: 66.254.114.238
The same queries run against the same Pi-hole from a system in the same subnet as the Pi-Hole responds with the correct IP for google.com but returns IP 0.0.0.0 for redtube.com.
Through further testing and research, I worked out it was an Ad blocking service on my router that was enabled for the client subnets that has been previously identified on other forums as interfering with DNS queries across subnets within the router. I do not remember enabling it (possible it was auto-updated as part of a recent firmware update) but once I turned off the Ad Blocking on the vlan for one of those clients, I could immediately see the traffic being logged in the PiHole logs.and I then started to get the responses I expected back, although for a Ubuntu device in a third subnet that I had also disabled Ad blocking for, it is still was resolving queries without hitting the Pihole even when I was targetting the PiHole Ip explicitly via dig and nslookup. I am looking into this further as I suspect it may be a mDNS function that is also enabled on that subnet