New screen assemblies seem to stop the keyboard backlight from working after logging in - And when I press the backlight brightness up or down on the touchbar, absolutely nothing happens, not even the brightness level icon appears on the screen. As soon as you put the original screen assembly back on and log in, the keyboard backlight is fine. This has been tested on numerous logic boards with over 10 different screen assemblies from different suppliers. When we have replaced the screen assembly we've noticed the unit will boot up fine, and at the login screen the Keyboard Backlight is visible, but as soon as you log into the OS and the desktop loads, the keyboard backlight will disappear and never ever appear again until the machine shuts down, and then starts back up (Logging out and back in does not work, the backlight will stay off until shutdown). We have worked on multiple 2019 pros no matter what board number it is, just the 2019 range.
First off I know it's a screen assembly issue as me and a colleague have been diagnosing this issue for the past two months.