Why occasionally do computers (or programs) take an IP address for a domain not on a separate primary DNS server, but from the default gateway x.x.x.1?

Question

Domain Name System

There are services in the company on servers within the working network.
In order to be able to enter from the external network, the external IP address of the gateway is registered on the gateway for the domains of these services.
And on a separate internal DNS server, internal IP addresses are registered. All computers in the working network have this internal DNS server configured.
But sometimes it happens that the service stops opening in the browser or the application that connects through the domain to the service stops working. DNS cache shows in such cases for the service the external IP of the gateway, i.e. as if the computer (DNS-client) sometimes looks not at the configured DNS server, but at the gateway.

Answer 1

At computers in a working network only one DNS server is registered? Does it never take off?

Answer 2

Since you have a service that is critically related to DNS, why not raise another server?