We can at least be able to speculate on some of these reasons.An iPhone X display a new diamond sub-pixel pattern, all the green sub-pixel appears in the line, while the red and blue sub-pixel alternating .You can here DisplayMate test in the image:
It seems that the electrical failure of several handsets causes the voltage to flow to all green subpixels.It extends from top to bottom indicating that it is something that appears at the edge of the display and sends incorrect voltages to a few lines of pixels If it's just a sub-pixel line, it will look slimmer, and it's close to the left and right sides of the phone, but it's hard to diagnose, a problem that often comes with ambitious devices using many new technologies. Last year, Samsung also had such a thing, but their 'death line' is pink. Even if only 0.001% of the screen shows an error, it is almost impossible to test the frequency of a few users eventually choose not to continue to use abnormal There are already users who report that their cellphones have been changed at the Apple Store, so this seems unlikely to be a software issue.