Huawei p40 pro why does the brightness change automatically?

I have a Huawei p40 pro and turned off the automatic brightness setting. I also turned off the "Improve readability in sunlight" option via the developer option. And still, after I set the brightness back to 100%, my mobile phone turns back to about half after a few minutes.

Why is that so?

And how can I finally prevent that?

Power saving mode active? Turn off.

The brighter a display, the more energy is used.

Have you activated the automatic light correction?

Thanks for your answer, but unfortunately my cell phone is in performance mode and the brightness drops again

As described in the text, the automatic light setting is deactivated.