It sounds a bit similar to this issue!?
After updating to Android 12, the rightness of the display of my #fp4 randomly decreases, but eventually temporarily reverts to correct brightness, repetitively. The period of time during which its brightness is less than it should be is always greater than the period of time during which its brightness is correct.
This has solely started to occur after the Android 12 update, and occurred approximately immediately subsequently, so I have cause to believe that it was caused solely by it.
There was a new button implemented with A12 called “extra dark/dim” is this on or off for you? You can find the button in the quick settings when you pull down from the top
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