This Article explains the change of Latency calculation.
Scope
FortiGate v7.4.4 and v7.6.x/
Solution
In the other versions, FortiGate will downgrade the server by increasing the Latency value by about 13-15 Seconds when a failure count occurs to choose the next available server to send the DNS request.
Instead of increasing the time to 15 seconds, It will incorporate the following:
- Server failure counts as a latency penalty at server selection time.
- FortiGate will Only adjust the RTT when a response has been received.