In short: A number of Pixel 2 and Pixel 2 XL users are finding that the Adaptive Brightness that Google debuted in Android Pie, is not actually working correctly. Many users are finding that when going from a dark room into a well-lit room or outside, the Pixel 2 or Pixel 2 XL, is not able to adapt to the change in brightness. Waiting a few seconds doesn't fix this either, users have to go into Quick Settings and adjust the slider. Which if you are outside, you likely can't find that slider in the first place, to be able to adjust it.
Background: Adaptive Brightness debuted in Android 9 Pie this year, and it is supposedly going to adapt to the lighting in the room that you are in, and work better than auto brightness. It uses machine learning to figure out how bright you want your screen in certain lighting conditions, which also conserves battery life, by not being overly bright when you don't want it to be, which is also pretty helpful. It has actually worked quite well, up until a few weeks ago. It would go pretty dim in darker rooms and then when you get outside, you could see the brightness going up til you can actually see the screen. So it does work, and work pretty decently, but as of lately, it hasn't worked at all.
Impact: There is a pretty obvious fix for this, which is to turn off Adaptive Brightness. But those that rather not worry about adjusting screen brightness all day, will find this to be annoying and an inconvenience. Hopefully Google will do something to fix this issue, it doesn't appear to be fixed in the recently released October 2018 security patch, so perhaps it'll get fixed in the November patch that should be on its way in a few weeks.