I had the same issue. It appears that this line ignore scale factor.
UIImage *maskedImage = [UIImage imageWithCGImage:masked];
So you should draw the image by yourself. Replace it by the following :
UIGraphicsBeginImageContextWithOptions(image.size, NO, 0.0);
CGContextRef context = UIGraphicsGetCurrentContext();
CGRect rect = CGRectMake(0, 0, image.size.width, image.size.height);
CGContextDrawImage(context, rect, masked);
UIImage * maskedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
Works fine by me.
EDIT
OR
UIImage * maskedImage = [UIImage imageWithCGImage:masked
scale:[[UIScreen mainScreen] scale]
orientation:UIImageOrientationUp];