Why is my auto brightness not working on my iPhone 7 Plus?

I have the same exact problem after replacing my cracked screen

I had this same issue, doing a DFU reset brought the auto brightness functionality back.

Long version: I tore my Touch ID ribbon cable while switching out the battery. I purchased a replacement screen with front camera/ proximity sensor already installed thinking it would make my install easier. Once completed, my Touch ID worked but I lost auto brightness even though the option was still in settings, it just didn’t work. The phone then updated itself to iOS 14.1 and the auto brightness option disappeared in settings. I was going to live without it but the screen was of a low quality and touch would randomly stop responding so I purchased an FX5 in-cell from Injured Gadgets along with their front camera because I didn’t want to risk messing up the OEM which I still plan on repairing by replacing the backlight. Anyway, after putting everything back together and doing a restart, auto brightness was still missing in settings but I can tell auto brightness was trying to work because the screen brightness was changing sometimes when I lift up and wake the phone. So I did a DFU reset and lo and behold, auto brightness is back, better than ever. Moral of the story, don’t trust that stuff on Amazon. Just learning about the batteries too, I got one of those 3800mah high capacity Amazon batteries in my phone and now I’m wary of the quality and of it is as advertised. Probably not, but it’s definitely better than the OEM battery it replaced which was at 85% capacity. Gets me through the day on a single charge. I’ll probably replace it with an IG battery in a few months.