Grey screen on macbook pro 13" retina early 2015 model

My macbook boots to a grey screen and can function normally while connected to an external display. I can adjust the brightness of the screen, everything else is working fine, including other components in the display component like the Apple logo at the back and the camera at the front. I can also see the information of the built-in retina screen in "system information".

I have tried SMC resets and NVRAM resets and both didn't work. I have also taken it to the Apple store and they ran a diagnostics and it said everything is fine. I am quite convinced that this is not a hardware problem, or at least it is not a severe problem that requires me to replace the whole display. Anyone have any ideas how I might fix this?