Under General Setup DNS servers are set to ISP dns servers
Under Services > DNS Forwarder is enabled with one Domain Override setup to forward to an internal DNS server IP.
This has been working as expected for years.
There have been no changes to the system in years, last update was to 2.3.4-RELEASE-p1 at least a year ago

It looks like the DNS domain forwarder stopped working a couple of days ago following a power outage.

Using Diagnostics > DNS Lookup , requests for the forwarder domain are resolved using configured external dns instead of using the internal domain override.

* Deleting and recreating the domain override made no difference.
* After changing the General DNS to include the internal DNS and rebooting, domains that the internal dns can resolve are still not being resolved
* From pfsense shell - "nslookup my.domain.com" fails to resolve, specifying the internal server : "nslookup 172.16.###.###" succeeds, so comms with server is fine, pfsense is just not using the internal server for resolution 

Right now I am getting around our most pressing issues by replacing any affected dns references with static IP addresses in aliases but there are still a couple of issues that require pfsense using the internal dns for resolution.

I am at a loss as to how to proceed.