The only way that is possible is if the CVT glass (small strip on the left side) is the cause, which would imply the glass is dirty or defective.
There can be no other cause as the only difference between the platen and an ADF scan/copy, is the glass itself, with the ADF the light stays still and the paper moves, off the platen the same light is used but it moves across the scan bed to do the page.
