Did you solve this? same problem, my code as below

private var MixassetWriter: AVAssetWriter?

guard let assetWriter = MixassetWriter else {
        return
    }
    self.MixisRecording = false
    
    assetWriter.finishWriting {
        self.MixassetWriter = nil
        if assetWriter.status == .completed {
            debugPrint(" completed file success")
            completion(assetWriter.outputURL)
        } else {
            debugPrint(" completed file failed = ", assetWriter.status.rawValue)
        }
    }