Try a forced-restart of your iPad - as this will often clear an isolated software issue:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.

If the issue persists, 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 may require repair or replacement of the iPad. If the line is visible in the screenshot, the something other than hardware will be the cause.
Note: When reviewing the image, rotating the iPad screen orientation by 90°, from Portrait to Landscape, will ensure that the image does not coincide with screen defects if present.
Take a screenshot on your iPad – Apple Support
If the test indicates that a hardware fault, a visit to the Genius Bar at your local Apple Store is recommended - where the technicians will be able to test and assess your iPad. This will likely inform your next steps:
Genius Bar Reservation and Apple Support Options - Apple
iPad Repair – Official Apple Support