Issue with driving Brushless Motor with a MOSFET Driver
You cannot and should not expect a one to one relation between the PWM duty cycle and the output voltage. The actual output voltage strongly depends on the load.

I guess that your load is the brushless motor you mention. Brushless motors contain some electronics to make the AC signal needed to drive the magnetic coils. These electronics can respond in different ways depending on the voltage they are supplied.

If you want a predictable voltage supplied to the motor then you will need feedback in your system to control the PWM duty cycle depending on the voltage. It might not be trivial to do this, there is a risk of instability.

But maybe the voltage doesn't matter, you might just want a fixed / controlled number of rpm at the motor. If your motor has a tacho output (like many fans in PCs have) then that could be used for feedback. Again, that might not be trivial to get working properly.