Those screens are able to "write" characters by strobing (turning on and off rapidly) individual pixels on the screen. The hardware driver that's responsible for that line is on all the time. It's unable to turn off, so you get a line on the screen. If it never came on, you'd get a blank line on the screen.
If it's new enough to be under warranty, contact the seller and see if they can swap it out and get a replacement from the factory. Most China vendors require you to take a picture of the defect before they act, so you are ahead one step if the US seller can swap it for you.
