Following the news that the latest iOS update can break phones with non-official replacement screens , repairers are encountering a different, more subtle problem: If you put a genuine Apple replacement display into an iPhone 8, 8 Plus or X, it'll no longer be able to adjust its brightness automatically. If Apple or one of its authorized partners were to put the same display in the same phone, though? No problem.
The aftermarket repair community has verified the behavior in phones from the US all the way to Australia. It's confirmed to be an issue with phones running iOS 11.1, 11.2 and 11.3, which led sources to suggest it's been a problem since the launch of the latest batch of iPhones last fall. I was able to confirm that even swapping the displays of two brand-new iPhones causes the ambient light sensor to stop working, despite it not being altered or touched in any way. Experiments have shown that the sensor is disabled by iOS during the boot process.