Yes, this is exactly why I went in the Apple store today. I was taking photo on a holiday vocation on a sunny day, and the phone suddenly got so dim that I can’t see anything on screen. I hard reboot the phone, the brightness seems to restore at first 10 second on the restart, but after 10 second it will go dim again, but not as dim as the time while I was taking photo. It went down like 20 percent brightness, whereas it went down for like 90 percent when taking photo. I had a iPhone 8 bedside me and used it to take video of the screen dimmed after 10 second of reboot. The brightness of iPhone 8 are much brighter than that of iPhone 12 Pro Max.
I do realize when it go down it brightness is when the phone starting to get hot, the temperature do not have to get to extremely hot, but you can feel the heat go up a bit.
Also to point out it has nothing to do with ->accessibility -> auto brightness or the or the reduce white point. As I read thru the comments, it seems when phone get a bit hotter it does go dim.
And just now, I was playing game war and order, while watching YouTube, and jumped in the discussion thread, and as I read thru the comment, the screen suddenly dimmed and phone started to heat up, then I went to the setting and turn of auto brightness and kept reading thru the comment (at this point of time I am not playing the game or YouTube) then the screen started to brighten up 3 time in within 10 second with brighter each time, last bright up was to it full brightness that make my eye felt uncomfortable. And I can feel the phone cools down as it go brighter.
I have showed the video of me taking the phone going dim while on holiday, and was replied this is may be a software issue and wait for later update. But it puzzled me that an iPhone 8 can operate under the same environment without heating up or going down in brightness, I truly felt a bit disappoint when this is my upgrade from 8 to 12 pro max, hopefully the next patch could fix this problem.
Case: 101316616810