Backlight flicker after screen replacement

I have fitted 2 screens for the same customer as they've been cracked. Both times after the screen is replaced it will flicker for around 10-20mins.

Device: IPhone 5s 16GB white.

OS : 8.1.3 UptoDate

Brightness reduced no longer flickers, only seems to occur on max brightness. Any idea why It would do it on this device only?

Iv'e fitted roughly 100+ Iphone 5s and iv'e never came accross this.

I'm thinking it could be a problem with the backlight power chip on the logic board? Or screen power chip?