Given the frequency of that answer, this could be a client excessively requesting DNS resolution, so this may be Pi-hole's recently introduced rate-limiting feature in action.
However, cached <domain> is (null) is unusual.
I'll see if I can get a developer to take a closer look.
In the meantime, try to find out which client would be responsible for those excessive queries, and stop it from doing so.
There is also a a chance you've closed a partial DNS loop that could have prompted those queries:
As you seem to have enabled Pi-hole's Conditional Forwarding, that would close a partial loop if your router would also be using Pi-hole as an upstream DNS server.
If that's the case, either disable CF in Pi-hole or remove Pi-hole as upstream in your router to avoid this.
And finally, you seem to have configured your router to distribute Pi-hole as local DNS server via DHCP, but your router is also adding its own IP:
*** [ DIAGNOSING ]: Discovering active DHCP servers (takes 10 seconds)
Scanning all your interfaces for DHCP servers
* Received 304 bytes from eth0:192.168.1.1
Offered IP address: 192.168.1.11
DHCP options:
Message type: DHCPOFFER (2)
server-identifier: 192.168.1.1
dns-server: 192.168.1.1
dns-server: 192.168.1.11
--- end of options ---
Currently, your clients will by-pass Pi-hole on occasions by picking your router for DNS.
See if you can disable this in your router.