In watchos, How to stay app in background while using avaudioengine.

Hello
My app record voice with Record_Engine class(AvaudioEngine).

Problem:
Even though i made my app go to background, app return to inactive state in few seconds(about 3 seconds after watch screen locked).

Example:

How can i leave my app background.
like built-in record app.

and my recorder class is here.

//Recorder.swift
class Record_Engine : ObservableObject {

   @Published var recording_file : AVAudioFile!

   private var engine: AVAudioEngine!
   private var mixerNode: AVAudioMixerNode!
   
   init() {
     setupSession()
     setupEngine()
   }
   
   
   fileprivate func setupSession() {
     let session = AVAudioSession.sharedInstance()
     do {
       try session.setCategory(AVAudioSession.Category.playAndRecord, mode: .default)
       try session.setActive(true)
     } catch {
       print(error.localizedDescription)
     }
   }
   
   fileprivate func setupEngine() {
     engine = AVAudioEngine()
     mixerNode = AVAudioMixerNode()
     mixerNode.volume = 0
     engine.attach(mixerNode)
     makeConnections()
     engine.prepare()
   }

   fileprivate func makeConnections() {
     let inputNode = engine.inputNode
     let inputFormat = inputNode.outputFormat(forBus: 0)
     engine.connect(inputNode, to: mixerNode, format: inputFormat)

     let mainMixerNode = engine.mainMixerNode
     let mixerFormat = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: inputFormat.sampleRate, channels: 1, interleaved: false)
     engine.connect(mixerNode, to: mainMixerNode, format: mixerFormat)
   }
   
   func startRecording() throws {
     let tapNode: AVAudioNode = mixerNode
       let format = tapNode.outputFormat(forBus: 0)
       self.recording_file = try AVAudioFile(forWriting: get_file_path(), settings: format.settings)

       tapNode.installTap(onBus: 0, bufferSize: 8192, format: format, block: {
         (buffer, time) in
         do {try self.recording_file.write(from: buffer)}
         catch {print(error.localizedDescription)}
       })

       try engine.start()
   }
}