172.16.8.3 is in private IP address space. I guess that IP address was a DNS server for some/all of the internal VPN-based network. The domain that is pointed to that is obscured in the screenshot, but I guess it is a private domain within the internal network? Without the override, the DNS requests will be forwarded up to the DNS server/s that pfSense is using by default. Maybe those are somehow able to resolve the names. Without more detail of your internal network and where the internal DNS server/s are, it is speculation from me.
If 172.16.8.3 is/was a gateway, presumably that gateway also had a DNS server or forwarder that could help resolve names. Maybe that functionality is no longer enabled on the gateway. Again, a bit of speculation from me.