By default Adaptive Brightness is On, which corrects the screen brightness based on lightning conditions. The sensor is on the left side of the speaker (portrait), under very bright conditions you should see it right under the glass. If you are holding your phone in a landscape mode there's a chance you will cover the sensor which therefore decreases screen brightness.
You can turn off adaptive brightness in Settings -> Display -> Adaptive Brightness
Yes i can confirm that issue on my device. (Android 8.1, the adaptive brightness is turned off.) The brightness shouldn't change at all, if the adaptive brightness is turned off.
I already saw that. Indeed, at brightness level 0 nothing change, but few level upper, I can see a little difference on a web page when I scroll with a picture and text (e.g. a news article). It looks like a brightness adaptation, same as on TV and PC. The brightness adapt not because of the light of the room, but because of the content on the screen. I don't know whether it is is a bug or not.