Basically in the DNS forwarder where you can specify a domain override, I had to also specify the LAN IP of pfSense (172.26.10.254 in my case) as the "Source IP" on the domain override configuration.
You usually have to do that when the DNS server that services the domain in question is over a VPN, because otherwise the source IP of the request (from the pfSense, across the VPN to the DNS server) will be some IP address of a VPN tunnel endpoint, or some internal tunnel address. The remote DNS server typically won't have a route back to that and so the reply to those DNS queries would never make it back.