Memory leak when adding custom NSOperations to NSOperationQueue

Question

I'm profiling (Leaks) my app (ARC) and it's showing several memory leaks which I can't figure out.

One object (ArchivingTasksManager) kicks off a method that creates many NSOperations within a for in loop, and adds them to a queue. Each NSOperation is a custom subclass (DownloadImageOperation) that I instantiate with an NSURL.

In ArchivingTasksManager:

- (void)archiveAllImages
{    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [self beginBackgroundTask];
        [self archiveImages];
    });
}

- (void)archiveImages
{
    self.queue.suspended = true;
    self.queue.maxConcurrentOperationCount = 1;

    for (Highlight *highlight in self.list.highlights) {
        for (Experience *experience in highlight.experiences) {
            for (NSURL *imageURL in [experience allImageURLsOfType:ExperienceImageType640x640]) {
                DownloadImageOperation *experienceImageDownload = [[DownloadImageOperation alloc] initWithImageURL:imageURL];

                [self.queue addOperation:experienceImageDownload];
            }

            NSURL *authorURL = [NSURL URLWithString:experience.author.image_250x250Url];

            if (authorURL) {
                DownloadImageOperation *authorImageDownload = [[DownloadImageOperation alloc] initWithImageURL:authorURL];

                [self.queue addOperation:authorImageDownload];
            }
        }

        MapSnapshotOperation *mapSnapshot = [[MapSnapshotOperation alloc] initWithHighlight:highlight];
        [self.queue addOperation:mapSnapshot];
    }

    [self.queue addObserver:self
                 forKeyPath:NSStringFromSelector(@selector(operationCount))
                    options:NSKeyValueObservingOptionInitial context:nil];

    self.totalOperations = self.queue.operationCount;
    self.queue.suspended = false;
}

In DownloadImageOperation:

- (instancetype)initWithImageURL:(NSURL *)imageURL;
{
    self = [super init];

    if (!self) {
        return nil;
    }

    _imageURL = imageURL;
    _targetURL = [[ArchivingOperation class] localImageURLFromRemoteImagePath:imageURL.absoluteString];

    NSURLSessionConfiguration *config = [NSURLSessionConfiguration ephemeralSessionConfiguration];
    _session = [NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];
    _downloadTask = [_session downloadTaskWithURL:imageURL];

    return self;
}

Here's what Instruments shows me in ArchivingTasksManager:

And here's what Instruments shows me in DownloadImageOperation: