MacBook Air brightness...

ItzxJAMD Said:

"what do you mean by 'failing screen' the screen brightness works if I do it manually, it turns on correctly... the screen works? so again what do you mean by failing screen?"

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A Failing Screen:

If it is dimming on its own, then maybe it cannot keep up with itself (power wise). If it flickers, then indeed that would mean a failing screen. But, you have mentioned nothing of it flickering.

A Few Other Thoughts:

A. External Monitor:

Try an external monitor. If you get this occurrence, then it is something configured in the macOS. If not, then it is the Screen itself.

B. New Admin User:

For Troubleshooting Purposes: Try creating a new administrator user. If you don't get this error, then it is something configured in this current user. Once no longer needed, just delete the new user.

C. Boot Into Safe Mode:

[Hold Down: the shift key upon boot until you get to the login screen]. In Safe Mode, Login Items do not run upon Login, particular System Preferences are set aside, and damaged system files (i.e. caches) are fixed. So, see if it then happens. After a few minutes, if it does not happen, then reboot normally, and login to the dame user giving you this issue. See if it has then been fixed.

ItzxJAMD Said:

"MacBook Air brightness...: my screen brightness has been automatically dimming by 20-30% each and every time the computer either wakes from sleep or returns from a lock screen after a few minutes of inactivity. This is despite having "automatically adjust brightness" turned off in Display Settings.[...]"

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Reinstall the MacOS;

Seems you have a failing screen. But, before concluding that, try performing a reinstall of the macOS. Sometimes, changes are made to your Mac (i.e. Updates), and then settings for particular items (i.e. Display Settings) become manipulated, technically.

Back Up First:

Important: Prior to reinstalling the macOS, backup your Mac, that way you have something to restore your Mac from, should anything go wrong with the reinstall.