Black frames in resulting AVComposition.

I have multiple AVAssets with that I am trying to merge together into a single video track using AVComposition.

What I'm doing is iterating over my avassets and inserting them in to a single AVCompositionTrack like so:

- (AVAsset *)combineAssets
{
  // Create a mutable composition
  AVMutableComposition *composition = [AVMutableComposition composition];
  AVMutableCompositionTrack *compositionVideoTrack =
    [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
  AVMutableCompositionTrack *compositionAudioTrack =
    [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
  
  // Keep track of time offset
  CMTime currentOffset = kCMTimeZero;

  for (AVAsset *audioAsset in _audioSegments) {
    AVAssetTrack *audioTrack = [[audioAsset tracksWithMediaType:AVMediaTypeAudio] firstObject];
    // Add the audio track to the composition audio track
    NSError *audioError;
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration)
                                   ofTrack:audioTrack
                                    atTime:currentOffset
                                     error:&audioError];
    if (audioError) {
      NSLog(@"Error combining audio track: %@", audioError.localizedDescription);
      return nil;
    }

    currentOffset = CMTimeAdd(currentOffset, audioAsset.duration);
  }

  // Reset offset to do the same with videos.
  currentOffset = kCMTimeZero;

  for (AVAsset *videoAsset in _videoSegments) {
    {
      // Get the video track
      AVAssetTrack *videoTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];

      // Add the video track to the composition video track
      NSError *videoError;
      [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoTrack.timeRange.duration)
                                     ofTrack:videoTrack
                                      atTime:currentOffset
                                       error:&videoError];
      if (videoError) {
        NSLog(@"Error combining audio track: %@", videoError.localizedDescription);
        return nil;
      }

      // Increment current offset by asset duration
      currentOffset = CMTimeAdd(currentOffset, videoTrack.timeRange.duration);
    }
  }

  return composition;
}

The issue is that when I export the composition using an AVExportSession I notice that there's a black frame between the merged segments in the track.

In other words, if there were two 30second AVAssets merged into the composition track to create a 60 second video. You would see a black frame for a split second at the 30 second mark where the two assets combine.

I don't really want to re encode the assets I just want to stitch them together. How can I fix the black frame issue?.