The posted image is indistinct, but given your description, you might be describing an inconsistency in the screen backlight. Given that your iPad Pro 12.9" M1 does not use a conventional edge-lit LCD - instead benefitting a miniLED backlight array - subtle inconsistencies in screen backlighting is uncommon.
About the Liquid Retina XDR display on iPad Pro - Apple Support
Are you using a screen protector with your iPad? If yes, you should remove the screen protector - and then re-evaluate the screen. Screen protectors can influence the observed screen image.
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 assess your iPad screen for issues. If repair is required, you will be in the best possible place to explore your available options:
Genius Bar Reservation and Apple Support Options - Apple