iPhone 13 pro blue screen

Helloe.

i sold an Iphone 13 pro around christmas. And now the buyer says he get (blue screen) on the phone some thimes. But he tested the mobile on site when he buyed it. What can ha happend and what can bee the problem here? The phone works just fine now when i use it and looking at it. I gona restore it and use it. and see if i get the blue screen or not. But some one knows what can bee the problem other wise?

/ Mark