[SOLVED] AVMutableVideoComposition: Overlay Static Text

In Swift for iOS, I have an array of AVURLAsset. I pass it through a function to stitch/merge the video assets together into one final video. For each video, my goal is to overlay text centered in the video frame.
When I play the video output, the video assets merge correctly, but no text overlays for any of the video assets. I tried following an existing answer, but no success. I also tried just overlaying text to a single video following Ray Wenderlich's tutorial, but for some reason the same end result :(.
Any guidance would be extremely appreciated..
func merge(videos: [AVURLAsset], completion: @escaping (_ url: URL, _ asset: AVAssetExportSession)->()) {
let videoComposition = AVMutableComposition()
var lastTime: CMTime = .zero

var maxVideoSize = CGSize.zero

guard let videoCompositionTrack = videoComposition.addMutableTrack(withMediaType: .video, preferredTrackID: Int32(kCMPersistentTrackID_Invalid)),
      let audioCompositionTrack = videoComposition.addMutableTrack(withMediaType: .audio, preferredTrackID: Int32(kCMPersistentTrackID_Invalid)) else { return }

let mainComposition = AVMutableVideoComposition()

let mainParentLayer = CALayer()
let mainVideoLayer = CALayer()
mainParentLayer.frame = CGRect(x: 0, y: 0, width: maxVideoSize.width, height: maxVideoSize.height)
mainVideoLayer.frame = CGRect(x: 0, y: 0, width: maxVideoSize.width, height: maxVideoSize.height)

mainParentLayer.addSublayer(mainVideoLayer)

var instructions = [AVMutableVideoCompositionInstruction]()
    
print("Main Parent Layer Frame: \(mainParentLayer.frame)")

for video in videos {
    
    if let videoTrack = video.tracks(withMediaType: .video)[safe: 0] {
        videoCompositionTrack.preferredTransform = videoTrack.preferredTransform
        print("Video Track Transform: \(videoTrack.preferredTransform)")

        do {
            try videoCompositionTrack.insertTimeRange(CMTimeRangeMake(start: .zero, duration: video.duration), of: videoTrack, at: lastTime)
            print("Video time range: Start = \(lastTime.seconds), Duration = \(video.duration.seconds) [\(lastTime.seconds + video.duration.seconds > 0 ? "Right" : "Wrong")]")

            if let audioTrack = video.tracks(withMediaType: .audio)[safe: 0] {
                try audioCompositionTrack.insertTimeRange(CMTimeRangeMake(start: .zero, duration: video.duration), of: audioTrack, at: lastTime)
            }

            lastTime = CMTimeAdd(lastTime, video.duration)

            let videoSize = videoTrack.naturalSize.applying(videoTrack.preferredTransform)
            let videoRect = CGRect(x: 0, y: 0, width: abs(videoSize.width), height: abs(videoSize.height))
            maxVideoSize = CGSize(width: max(maxVideoSize.width, videoRect.width), height: max(maxVideoSize.height, videoRect.height))

            let textLayer = CATextLayer()
            textLayer.string = "TESTING TESTING TESTING"
            textLayer.foregroundColor = UIColor.white.cgColor
            textLayer.font = UIFont(name: "Helvetica-Bold", size: min(videoRect.height / 10, 100))
            textLayer.shadowOpacity = 0.5
            textLayer.alignmentMode = .center
            textLayer.contentsScale = UIScreen.main.scale
            textLayer.isWrapped = true

            let textHeight: CGFloat = min(videoRect.height / 10, 120)
            let textWidth: CGFloat = videoRect.width
            let xPos = (videoRect.width - textWidth) / 2
            let yPos = (videoRect.height - textHeight) / 2
            textLayer.frame = CGRect(x: xPos, y: yPos, width: textWidth, height: textHeight)
            textLayer.zPosition = 1
            print("Text Layer Frame: \(textLayer.frame) [\(textLayer.frame.width > 0 && textLayer.frame.height > 0 ? "Right" : "Wrong")]")

            let parentLayer = CALayer()
            parentLayer.backgroundColor = UIColor.green.cgColor // Temp background color for debugging
            parentLayer.frame = videoRect
            parentLayer.addSublayer(textLayer)
            
            print("Video Layer zPosition: \(mainVideoLayer.zPosition), Text Layer zPosition: \(textLayer.zPosition) [\(textLayer.zPosition > mainVideoLayer.zPosition ? "Right" : "Wrong")]")

            let videoCompositionInstruction = AVMutableVideoCompositionInstruction()
            videoCompositionInstruction.timeRange = CMTimeRangeMake(start: lastTime - video.duration, duration: video.duration)
            let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack)
            videoCompositionInstruction.layerInstructions = [layerInstruction]
            instructions.append(videoCompositionInstruction)

            parentLayer.zPosition = 0
            mainParentLayer.addSublayer(parentLayer)
            
            print("Parent Layer Frame: \(parentLayer.frame), Background Color: \(parentLayer.backgroundColor.debugDescription)")
            print("Text Layer Frame: \(textLayer.frame)")
            
        } catch {
            print("Failed to insert track: \(error.localizedDescription)")
            return
        }
    }
}

mainParentLayer.frame = CGRect(x: 0, y: 0, width: maxVideoSize.width, height: maxVideoSize.height)
mainVideoLayer.frame = mainParentLayer.frame

mainComposition.renderSize = maxVideoSize
mainComposition.instructions = instructions
mainComposition.frameDuration = CMTime(value: 1, timescale: 30)
mainComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: mainVideoLayer, in: mainParentLayer)

print("Final Main Parent Layer Frame: \(mainParentLayer.frame)")
print("Number of Sublayers in Main Parent Layer: \(mainParentLayer.sublayers?.count ?? 0)")

let outputUrl = NSURL.fileURL(withPath: NSTemporaryDirectory() + "merged" + ".mp4")

print("Pre-Export Main Parent Layer Frame: \(mainParentLayer.frame)")
print("Pre-Export Number of Sublayers in Main Parent Layer: \(mainParentLayer.sublayers?.count ?? 0)")
if let sublayers = mainParentLayer.sublayers {
    for (index, layer) in sublayers.enumerated() {
        print("Layer \(index): \(layer), Frame: \(layer.frame), zPosition: \(layer.zPosition)")
    }
}

guard let exporter = AVAssetExportSession(asset: videoComposition, presetName: AVAssetExportPresetHighestQuality) else { return }

exporter.videoComposition = mainComposition
exporter.outputURL = outputUrl
exporter.outputFileType = .mp4
exporter.shouldOptimizeForNetworkUse = true

if let videoComposition = exporter.videoComposition {
    print("Export Video Composition Render Size: \(videoComposition.renderSize)")
    print("Export Video Composition Frame Duration: \(videoComposition.frameDuration)")
    print("Export Video Composition Instructions Count: \(videoComposition.instructions.count)")
}

exporter.exportAsynchronously {
    DispatchQueue.main.async {
        if let outputUrl = exporter.outputURL, exporter.status == .completed {
            completion(outputUrl, exporter)
        } else if let error = exporter.error {
            print("Export failed: \(error.localizedDescription)")
        }
    }
}
play(video: exporter.asset)
}