Possible fix: turn off “Zoom”
I have been struggling with this problem for several months on my iPhone 11 Pro. I think the problem started around my update to iOS 14, 14.1, or 14.2. I manage my family’s iPhones as well (iPhone 7, iPhone 7, and iPhone XS) but none of them have complained of this problem despite also running iOS 14.2.
On my iPhone 11 Pro, I turned off brightness auto adjust, which made it very tedious to have to manually change brightness throughout the day. It also seemed that I could not get the screen bright enough manually when in sunlight. It seemed that I could not get the screen as bright as I once was able to. And then I stumbled on a fix that worked for me. Perhaps it will for you.
Go to:
Settings
Accessibility
Zoom
Turn Zoom off if it’s on
Try adjusting brightness manually to test and/or go to:
Settings
Accessibility
Display & Text Size
at the bottom of this settings page, turn Auto-Brightness on
Hope this helps others.
Good luck.