So I've been trying to figure this out for about a week now. My top screen just mysteriously stopped working. It still works perfectly fine in ds mode, but as soon as I exit, it's just black. No backlight or anything.
Does it immediately go to a black screen or does it slowly fade into black like this?: https://dl.dropboxusercontent.com/s/blmd30aivg2c2pj/3ds.mp4
Happens about 29 seconds into the video.