It seems to be operating exactly as it was designed to operate.
The Corsair AX850 will operate in fanless mode (i.e. 0 RPM) until you reach 20% load (170 Watts) on the PSU then the fan will kick in.
From 170 Watts to 425 Watts the PSU fan will operate in low RPM mode.
Above 425 Watts the fan speed will ramp up exponentially.