Dark spots showing up on my new iPad Pro (12.9 inch, 5th gen)

I have this black blurry black line on one side of the screen... (see photo below) and does anybody know what caused it?

The thing is that it disappeared the next day...

But should I take it to Apple anyways? (I have Apple Care)

PLEASE advice. I read some other reports but no one followed up on the results of their question.

Thanks