IPad Pro 11"

Begin with a forced restart - as this will often clear an isolated software issue:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

If the problem persists, you perhaps have a screen hardware issue. A reliable method to confirm a screen or other hardware fault is to take a screenshot when the fault is evident - and then review the captured screenshot image...

When reviewed, if the line is not present in the screenshot image, then you have demonstrated that you have a hardware fault that, sooner or later, may become sufficiently severe that repair or replacement of the iPad are the only options. If the line is visible in the screenshot, the something other than hardware will be the cause.

Note: When reviewing the image, rotate the iPad screen orientation by 90°, from Portrait to Landscape, to ensure that the image does not coincide with screen defects if present.

Take a screenshot on your iPad – Apple Support

If a hardware fault is suspected, you would be best advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:

Genius Bar Reservation and Apple Support Options - Apple