Screen ghosting after power button repair

[amp]

I hope someone might be able to help me out. My 4S recently developed an unresponsive power button so I ordered a replacement power button/light proximity sensor flex cable on eBay and swapped it out with the old one. However, now I see that I get quite a bad 'ghosting' type problem on the screen. It's particularly obvious on images/videos with dark backgrounds. I've taken a screenshot of the screen and opened the picture on my laptop but the ghosting isn't visible so I assume it's a display issue rather than a graphics problem. I was careful when taking the phone apart and reassembling when replacing the flex cable and have also again taken the phone apart to see if anything looks damaged but as far as I can tell it's all fine. Has this happened to anyone else and is there anything I can do to fix it? Many thanks, Andy.