Sudden random changes in screen brightness

I use an iPhone 12 mini which I am generally speaking really happy with!

There are, however, a couple of small issues I have experienced concerning the screen brightness.

These problems were there under iOS 14 and are still there after updating to iOS 15.1.

I have disabled True Tone and Auto brightness.

1) The first problem is that sometimes the screen brightness is reduced for no apparent reason. In these cases the brightness slider in the control center remains at the maximum level, but the screen is definitely not at maximum brightness. One time I could move the slider down to ca 50% before the brightness was reduced further, meaning the screen brightness was reduced to 50% while the slider was still at maximum.

Whenever this has happened, it resolves itself after a while.

2) I have noticed small random changes in brightness, almost flickering like. I found these to be most visible when viewing darker things as for example the Facebook app in dark mode, the dark home page of Chrome or viewing videos in the Youtube app.

One time, this flickering was really prominent while using the Youtube app. I did some screen recordings, however, the flickering did not show in the recording, meaning it really is random changes in brightness.

Any ideas what the reason could be for these issues? I am concerned that there is something wrong with the screen in my phone.

Thanks for any answers!