I have it on my iPhone 7 Plus. I never noticed it before! I just checked out of curiosity and sure enough, first email I clicked on, I see the twitchy flashing at the top as shown on the video. I tend to look to the center of the display or look at something else entirely when opening email which is why I never noticed before.
Since it’s on an older model like mine which is pretty well tested, I wouldn’t worry about it being a hardware problem on your new iPhone X.
Exact same scene here (7 Plus / iOS 11.1). I almost never use the the Mail app, but opened and force-closed it a few times for this test. Indeed, when I first open an email message upon launching the app, the top of the window flickers.
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I have seen it too. It always occurs the first time you open an email upon opening the email app. If you leave the mail app open in the background and you go back to it and open and email, you don't see the flicker. You have to force close the email app. open it, and open any email, the first one will have the flicker. None of them after that.
This summarizes it exactly.