Chosen Solution
After I replaced my iPhone 7 Plus’s front camera and proximity sensor (because I accidentally ripped the first one), the auto brightness has not worked since. I have went into settings to try to manually turn it on and the option isn’t even there.
The auto brightness option is now in the Accessibility menu. Go to Settings/Accessibility/Display Accomodations to find it. Your phone may require to be reset in order to recognize the new proximity sensor. Try a hard reset or even a full backup/restore first. Failing that, given that the replacement flex does not work either, you may have an issue on the logic board.
I have the same exact problem after replacing my cracked screen
