Thin black line across my iPad Pro screen

Perform a forced-restart of your iPad - 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 issue survives a forced-restart, your iPad may have a fault. 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 lines are 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 lines are 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 are hardware fault is suspected, your iPad will need to be assessed for repair. 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 - and discuss your available options:

Genius Bar Reservation and Apple Support Options - Apple

If you are reviewing the screenshot on the affected iPad, you must rotate the iPad by 90° (from landscape to portrait orientation or vice versa) when reviewing the captured screenshot. If you don’t change the screen orientation, a screen defect will align with the screenshot image.

If you post the screenshot here, we can be certain of the diagnosis.