Garmin upgrade problem
We did the normal software upgrade to our Garmin 2797 a couple of months ago, not maps. On our test camping run of the new charging system, we found the Garmin would constantly go to 20% brightness, no matter what the settings were at. Couldn't see it at all in sunlight. Never did that before. Once we got settled in at our campsite, I did a new software update, and the problem went away.
So, if you have a Garmin and did an update recently, you may want to do it again. If you have the screen dimming issue, you probably should try the update.