The green line on this screen is not lost, no matter what the user is doing. Indeed, this issue does not affect the performance of the phone, but it is still annoying.
Apple has not answered when asked about this issue. However, it seems that this green line can not be solved by restarting or other solutions that are typically used to fix small software problems. There is a possibility, this problem is in hardware.
It remains unknown how many people have experienced this problem. Fortunately users who experience this problem is not much.
Additionally, if users can not resolve this issue in the normal way, they can request a replacement unit. Not just a green line, iPhone X has other issues related to screen response rate when used in cold weather.
Not strange if the iPhone X still has a little problem, considering this is the first time Apple uses OLED screen. The screen was even made specifically by Samsung.