[Geoff Bacon]

Sounds like adaptive brightness is a possibility

To turn it off (taken from another web site)

1. Open Settings on Windows 11.
   
2. Click on System.
   
3. Click the Display page on the right side.
   
4. Under the “Brightness & color” section, clear the “Automatically adjust contrast based on the display content to help improve battery” option.
   

Note: this description isn't talking about ambient light, it is talking about screen content e.g. in landscape mode, you might have large area of white causing the brightness to change.

Your description implies that you are switching between portrait and landscape mid way through a gig.
Can you adjust your "songs" so that they are all portrait or all landscape? This would eliminate rotating the laptop in the gig.

Geoff
PS I'm using Android on my tablet.