I had what I think is the same problem: my pihole is set to resolve unknown domains from my router, and the router is configured to provide IP addresses various .lan hostnames. However after upgrading, the pihole was successfully resolving the IP form the router (as seen in the logs), but then replying with "0.0.0.0" to all requests.
I found the expert setting DNS > DNS domain settings > Pi-hole domain name, which was set to "lan". I cleared this, and the problem was resolved.