iPhone 8 shows horizontal lines when turns on

Hello everyone

I am using an iPhone 8 with iOS 12.1.4 and recently I found that when I turn my phone on, when the screen with the Apple logo comes, some horizontal white dotted lines are shown in the black background. These cannot be seen if I am in a bright environment. The lines are so thin and disappear quickly. The position that the lines lay is random. So I guess the problem is not with the display.

What is the problem?