This seems to be a device specific bug on only iPhone 6S Plus.
Steps:
AVPlaye
I tried Nate4436271's answer and it worked for me, I mean, had the same issue and got rid of the gray lines, but setting the shouldRasterize property to true, made my video lose image quality. So, I set the videoGravity of my playerLayer to AVLayerVideoGravityResizeAspectFill and I got rid of the lines, while maintaining image quality :)
Here is what I did:
playerLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
AspectFill was within my needs and solved the problem. You may wish to evaluate whether other options are better for what you need, like AspectFit or Resize.
Hope this helps :)