Exporting AVAsset second time makes video blank

I am stitching multiple video files into one using AVMutableComposition() adding tracks like this:

let compositionVideoTrack = mainComposition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
let soundtrackTrack = mainComposition.addMutableTrack(withMediaType: .audio, preferredTrackID: kCMPersistentTrackID_Invalid)

private var insertTime = CMTime.zero
     
for videoAsset in arrayVideos {
       try! compositionVideoTrack?.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .video)[0], at: insertTime)
       try! soundtrackTrack?.insertTimeRange(CMTimeRangeMake(start: .zero, duration: videoAsset.duration), of: videoAsset.tracks(withMediaType: .audio)[0], at: insertTime)
            
       insertTime = CMTimeAdd(insertTime, videoAsset.duration)
}

then exporting them using AVAssetExportSession(asset: mainComposition, presetName: AVAssetExportPresetMediumQuality) into .mov file.

That saves the stitched video to url, which I can access using AVAsset and display to user. After that I am trying to add image overlay to the video and export it again.

In this second method, I instantiate the AVAsset from url AVAsset(url: fileUrl). And create new AVMutableComposition(). I add video and audio track to the composition from the asset:

    compositionTrack = composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)
    compositionTrack.insertTimeRange(timeRange, of: asset.tracks(withMediaType: .video)[], at: .zero)
...

Then I add overlay to the video using layers and AVVideoCompositionCoreAnimationTool() like this:

 let videoLayer = CALayer()
 videoLayer.frame = CGRect(origin: .zero, size: videoSize)
 let overlayLayer = CALayer()
 overlayLayer.frame = CGRect(origin: .zero, size: videoSize)
        
 overlayLayer.contents = watermark.cgImage
 overlayLayer.contentsGravity = .resizeAspect
        
 let outputLayer = CALayer()
 outputLayer.frame = CGRect(origin: .zero, size: videoSize)
 outputLayer.addSublayer(videoLayer)
 outputLayer.addSublayer(overlayLayer)
        
 let videoComposition = AVMutableVideoComposition()
 videoComposition.renderSize = videoSize
 videoComposition.frameDuration = CMTime(value: 1, timescale: 30)
 videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: outputLayer)
        
 let instruction = AVMutableVideoCompositionInstruction()
 instruction.timeRange = CMTimeRange(start: .zero, duration: asset.duration)
 videoComposition.instructions = [instruction]
 let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: assetTrack)
 layerInstruction.setTransform(assetTrack.preferredTransform, at: .zero)
 instruction.layerInstructions = [layerInstruction]

and then I export the video same way as in the first export.

The issue is when I am combining this. If I only export some sample video using the second method, overlay to the video is added and everything is as expected. If I stitch the videos using the first method, videos are then stitched perfectly. However when I combine these methods, the resulting video is black blank screen (audio and the overlay is good, the resulting duration of video also fits).

The issue had to do probably something with AVVideoCompositionCoreAnimationTool(). I was able to solve this by using another technique for adding overlay to video in the second function. Instead of stacking layers using AVVideoCompositionCoreAnimationTool(), I've used a CIFilter like so:

let watermarkFilter = CIFilter(name: "CISourceOverCompositing")!
let watermarkImage = CIImage(image: watermark)!
let videoComposition = AVVideoComposition(asset: asset) { (filteringRequest) in
    let source = filteringRequest.sourceImage
    watermarkFilter.setValue(source, forKey: kCIInputBackgroundImageKey)
    let widthScale = source.extent.width/watermarkImage.extent.width
    let heightScale = source.extent.height/watermarkImage.extent.height
    watermarkFilter.setValue(watermarkImage.transformed(by: .init(scaleX: widthScale, y: heightScale)), forKey: kCIInputImageKey)
    filteringRequest.finish(with: watermarkFilter.outputImage!, context: nil)
    }