Yes, I've noticed the same issue on my iPhone 5, which started happening in the last O.S. update or two (I'm currently running 6.1.3). It's always after I've begun using the Google Maps app, and after maybe 1 minute, whether I have Auto-Brightness enabled or not, my screen automatically dims to its minimum value. I can raise the brightness of course under settings, but then it just happens again 1-2 minutes later! I've closed the Google Maps app but it continues to be an issue, but seems to go away after shutting the iPhone down and then restarting.
I've never noticed this happening other than after using the Google Maps app, and never with any other app. Very strange, but highly annoying (and dangerous, as I'm typically driving when this happens, and it takes some time to get the brightness back up).