CGAffineTransformMakeRotation bug when masking - objective-c

Question

I have a bug when masking a rotated image. The bug wasn't present on iOS 8 (even when it was built with the iOS 9 SDK), and it still isn't present on non-retina iOS 9 iPads (iPad 2). I don't have any more retina iPads that are still on iOS 8, and in the meantime, I've also updated the build to support both 32bit and 64bit architectures. Point is, I can't be sure if the update to iOS 9 brought the bug, or the change to the both architectures setting.
The nature of the bug is this. I'm iterating through a series of images, rotating them by an angle determined by the number of segments I want to get out of the picture, and masking the picture on each rotation to get a different part of the picture. Everything works fine EXCEPT when the source image is rotated M_PI, 2*M_PI, 3*M_PI or 4*M_PI times (or multiples, i. e. 5*M_PI, 6*M_PI etc.). When It's rotated 1-3*M_PI times, the resulting images are incorrectly masked - the parts that should be transparent end up black. When it's rotated 4*M_PI times, the masking of the resulting image ends up with a nil image, thus crashing the application in the last step (where I'm adding the resulting image in an array).
I'm using CIImage for rotation, and CGImage masking for performance reasons (this combination showed best performance), so I would prefer keeping this choice of methods if at all possible.
UPDATE: The code shown below is run on a background thread.
Here is the code snippet:
CIContext *context = [CIContext contextWithOptions:[NSDictionary dictionaryWithObject:[NSNumber numberWithBool:NO] forKey:kCIContextUseSoftwareRenderer]];
for (int i=0;i<[src count];i++){
//for every image in the source array
CIImage * newImage = [[CIImage alloc] init];
if (IS_RETINA){
newImage = [CIImage imageWithCGImage:[[src objectAtIndex:i] CGImage]];
}else {
CIImage *tImage = [CIImage imageWithCGImage:[[src objectAtIndex:i] CGImage]];
newImage = [tImage imageByApplyingTransform:CGAffineTransformMakeScale(0.5, 0.5)];
}
float angle = angleOff*M_PI/180.0;
//angleOff is just the inital angle offset, If I want to start cutting from a different start point
for (int j = 0; j < nSegments; j++){
//for the given number of circle slices (segments)
CIImage * ciResult = [newImage imageByApplyingTransform:CGAffineTransformMakeRotation(angle + ((2*M_PI/ (2 * nSegments)) + (2*M_PI/nSegments) * (j)))];
//the way the angle is calculated is specific for the application of the image later. In any case, the bug happens when the resulting angle is M_PI, 2*M_PI, 3*M_PI and 4*M_PI
CGPoint center = CGPointMake([ciResult extent].origin.x + [ciResult extent].size.width/2, [ciResult extent].origin.y+[ciResult extent].size.width/2);
for (int k = 0; k<[src count]; k++){
//this iteration is also specific, it has to do with how much of the image is being masked in each iteration, but the bug happens irrelevant to this iteration
CGSize dim = [[masks objectAtIndex:k] size];
if (IS_RETINA && (floor(NSFoundationVersionNumber)>(NSFoundationVersionNumber_iOS_7_1))) {
dim = CGSizeMake(dim.width*2, dim.height*2);
}//this correction was needed after iOS 7 was introduced, not sure why :)
CGRect newSize = CGRectMake(center.x-dim.width*1/2,center.y+((circRadius + orbitWidth*(k+1))*scale-dim.height)*1, dim.width*1, dim.height*1);
//the calculation of the new size is specific to the application, I don't find it relevant.
CGImageRef imageRef = [context createCGImage:ciResult fromRect:newSize];
CGImageRef maskRef = [[masks objectAtIndex:k] CGImage];
CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
CGImageGetHeight(maskRef),
CGImageGetBitsPerComponent(maskRef),
CGImageGetBitsPerPixel(maskRef),
CGImageGetBytesPerRow(maskRef),
CGImageGetDataProvider(maskRef),
NULL,
YES);
CGImageRef masked = CGImageCreateWithMask(imageRef, mask);
UIImage * result = [UIImage imageWithCGImage:masked];
CGImageRelease(imageRef);
CGImageRelease(masked);
CGImageRelease(maska);
[temps addObject:result];
}
}
}
I would be eternally grateful for any tips anyone might have. This bug has me puzzled beyond words :) .