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I have the same issue in iPhone 8 with ios 12.1. A random line of white or gray pixels that appears half a second while booting. I do not know is it quite common?

I have the same issue, and after update to ios 12.1.1 still exist, Are you still face the same ?
 
Yes I still have the problem. But I have tried to ignore it since the phone works perfectly

I will try to ignore it, But I am surprised by the lack of reaction from Apple, where there are a large number of users of iPhone 8 &8plus suffer from the same problem and there is no solution or not even response from the Apple side
 
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