My old S10 did that really well and it makes the screen look amazing even under bright sunlight.
I just recently noticed that my S21 does not, at all!
Even outside under the sunlight on a bright day, the screen just maintained its regular maximum brightness, which of course was not enough!
Restarting the phone did not fix it.
I had not installed any third party apps that would interfere with phone brightness settings and always had adaptive brightness mode on.
I am not covering any sensors I am aware of and merely using a film screen protector. (normal brightness adjustment works fine so that is out of the question)
Not sure what could be the issue here, please share your thoughts, anything would help, thanks!