I found the answer myself by commenting out different parts of my code until I could pinpoint the part of my code that caused this behavior.
The problem was caused by the following code segment, which I used to change the color of icons..
UIGraphicsBeginImageContextWithOptions(image.size, NO, image.scale);
CGContextRef context = UIGraphicsGetCurrentContext();
[color setFill];
CGContextTranslateCTM(context, 0, image.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextClipToMask(context, CGRectMake(0, 0, image.size.width, image.size.height), [image CGImage]);
CGContextFillRect(context, CGRectMake(0, 0, image.size.width, image.size.height));