My name is Jonathan Deives. I'm an Independent Advisor and I'll be glad to help you today.
The default refresh rate setting is 60 Hertz and therefore may return to normal when the PC is restarted.
Do you use any programs that depend on 40 Hz?
Check the settings of the monitor itself, some models have Hz changes through the settings.
You can also use a 3rd tool to create custom display profiles and maintain 40Hz even after restarting.
If you have more questions, I'll be happy to help.