Never seen that on an iPad, but it did happen on an iMac I owned once. It was the beginning of the end for that screen. It got progressively worse over time. Not saying that's your case here, but it's a possibility.
Any time there’s a vertical or horizontal line, it’s nearly 100% a hardware problem, often due to drops or bumps. The display is controlled by a matrix of rows and columns. If the controller can’t communicate with the LCD, it will fail as a line.