I tried to add a picture to make it clearer but it wouldn’t load. Glad you figured it out.
It is supposed to adjust back to full bright in the day automatically. To summarize it:
The + and - button allow you to adjust how bright the dashboard is at night, but there is an override mode to turn the dashboard back to full brightness (by pressing the + mode all the way passed max brightness).
This is useful in situations where the screen may dim when you don’t want to. (For example at night dawn when it gets dark enough for the auto light to trigger but it is still too bright outside for the dimmed screen to be readable).
Pressing “ -“ turns the override function off.
I am guessing your override function was turned how by mistake or by the dealer in the garage. You shouldn’t have to touch it again unless in special situations like I mentioned above.
Every car dashboard that relies on the lights to make the dashboard visible during the day (either digital or analog where the numbers need to be lit up to be visible in the day) have this function in one form or another, but it is very poorly known. My Mazda 6 2004 had it.