Like the ms009986 said I already posted the same thing 93 times and guess what, I'll post 93 times if necessary.
Because i spend 7 months trying to figure out with apple a solution for this problem, was a tons of phone calls and a tons of store appointments.
First they changed my screen and nothing, after they give a brand new phone and nothing again, at this point I was VERY UPSET because the reason I decided spend a thousand dollars in a phone was looking for a perfect experience.
So I decide to call one last time and let they know how upset I was and that I demand a phone from apple that don't change the brightness for itself, the senior guy ( after 7 months ) told me that this is a built in feature you can't control and after the iPhone 6 every iPhone has this built in feature, and he did more, he said that all brands has the same built in feature, guess what I upgraded to a android phone ( note 9 ) and the screen is always at the brightness level that I stipulate.
Don't wasted your time and patience with this company they don't deserve it.