Maybe the backlight is bad or has a very random short. It's not an LED screen, so it's dependent upon a secondary light source (a backlight, which oddly enough is probably an LED, so it should not burnout. Hence, there's a malfunction of some sort). It seems like that source (said LED) is starting to malfunction on some vehicles, for one reason or another.

I'd insist upon having it replaced prior to the end of your warranty. I'm sure the NAV must be replaced as a unit and is non-serviceable by the dealer. It probably costs close to what buying an extended warranty does: $2,000+ ???

I'm almost tempted to install a bracket for my Flip camcorder that starts video recording the first 1-minute of each vehicle start, as this is when everyone is having trouble; trouble of which cannot be replicated by the dealers. How nice for BMWNA!