Deleted

maybe you need to shut down your pc properly?
or you have a screensaver programm that sets them back to default?

Launch Command Prompt (Admin) > type powercfg –restoredefaultschemes >hit Enter to reset the power plan settings to default.
Exit command prompt to remove all customized power plans.
Create a new power plan and customize it according to your needs.

This explains how to change the different power plans. Is this where you are changing it?

https://www.windowscentral.com/how-manage-custom-power-plans-windows-10

If you are talking about windows then most probably this data is stored in windows registry. Unsure if you can make one particular branch of registry read-only, but you can probably change access rights to it so that it could not be changed (I never tried it myself, so I'm not sure it's possible.)

On some computers, it is possible that a program installed by the computer manufacturer (or installed with a hardware driver) manages the way the screen turns off, and this despite the Windows settings. Try to see if you have a program somewhere in your Start Menu to manage energy consumption or display (especially one with the name of the computer manufacturer).

Somewhere in Windows registry if I recall. That being said, there's a few ways where you can edit the inactivity display power off instead of registry.

This should help - https://www.online-tech-tips.com/windows-7/does-your-windows-7-computer-display-turn-off-every-15-minutes/

https://www.online-tech-tips.com/wp-content/uploads/2009/11/change-plan-settings.png

PowerSchemeEd
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