iPhone 13 Pro Max Why the screen brightness comes down every time ?

[Micka88]

I set the screen brightness to some high level but after some time it jumps to very low brightness and I have to put is back again. Night shift is switched off, true tone on. Why is it happening ?

 

[JGWolven]

Turn Auto-Brightness off in Settings -> Accessibility -> Display & Text Size -> Auto Brightness at the bottom