Multi face recognition settings not work

The code looks like this:
But the printed identifier has not changed, is it always one? Is there anything I set up?

override func viewWillAppear(_ animated: Bool) {

super.viewWillAppear(animated)


let configuration = ARFaceTrackingConfiguration()

print(ARFaceTrackingConfiguration.supportedNumberOfTrackedFaces)

configuration.maximumNumberOfTrackedFaces = 2

faceSCNView.session.run(configuration)

}


extension GameViewController : ARSCNViewDelegate {


func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {

if anchor is ARFaceAnchor {

print( anchor.identifier , anchor.name ?? "")

}

}


func renderer(_ renderer: SCNSceneRenderer, didUpdate node: SCNNode, for anchor: ARAnchor) {

guard

let faceAnchor = anchor as? ARFaceAnchor

else { return}

print(faceAnchor.identifier , faceAnchor.blendShapes[.mouthClose] ?? 0)

}

}


extension GameViewController : ARSessionDelegate {


func session(_ session: ARSession, didAdd anchors: [ARAnchor]) {

print(anchors.count)

for anchor in anchors where anchor is ARFaceAnchor {

let faceAnchor = anchor as! ARFaceAnchor

print("Session AddAnchor", faceAnchor.identifier , faceAnchor.blendShapes[.mouthClose] ?? 0)

}

}


func session(_ session: ARSession, didUpdate anchors: [ARAnchor]) {

for anchor in anchors where anchor is ARFaceAnchor {

let faceAnchor = anchor as! ARFaceAnchor

print("Session Update", faceAnchor.identifier , faceAnchor.blendShapes[.mouthClose] ?? 0)

}

}

}