It looks like you can have it repaired. I'm not sure if the pad is something that can be repaired though since it's basically 80% of the tablet itself.
You can try to get it looked at/sent in here: https://www.wacom.com/en-us/support/contact-support/returns-and-repairs
They'd probably suggest getting a new one, or if it doesn't effect your drawing, I wouldn't worry about it too much.
Most logical reason is something was brushed against it really fast by you or someone else and didn't notice it, or a more rare scenario is something hard was stuck to your pen tip and scratched it as you glided across. Something like a piece of hard plastic/food/etc. You'd have to be applying a bit of pressure though which is why I think the first scenario is more likely.
Since it's only semi-deep, something as harmless as the zipper on a jacket gliding over it could have scratched it tbh. My tablet is ancient, but I won't get a new one til she dies. She's got so many scratches on her, but none of them are deep enough to effect how the pen glides and whatnot.