In Unity, why can I switch the Camera to WorldView with a button press but cannot do it in the Start or Update loop while also using Face Manager?

Question

I'm working on a Unity AR game in which I'd like to start out using the Face Manager to track Face Pose on the World-Facing Camera. I know for a fact that you can track Face Pose with the World Camera because I've seen it in the ar-foundation-samples project. The one caveat to that is that you have to start in User-Facing Mode first, then press a button to switch to World-Facing mode once the app is running. I'd like for it to start out on World-Facing camera, but every time I try to do this with the Face Manager running, it doesn't work; the camera doesn't come on at all. My question is, why can I switch my face pose tracking from User-Facing to World-Facing with a button click, yet I can't do the same via the Start() or Update() methods?

public class FaceController : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        /*This won't switch the camera*/        
        camManager.requestedFacingDirection = CameraFacingDirection.World;
    }

    private void OnEnable()
    {
        
    }

    public ARCameraManager camManager;
    public ARSession aRSession;
    bool trigger;
    bool trigger2;

    public void SwitchMode()
    {
        /*This will switch the camera when called by a button press */
        camManager.requestedFacingDirection = CameraFacingDirection.World;
    }

    // Update is called once per frame
    void Update()
    {
      
      
    }
}

Answer 1

So I discovered that you could only switch cameras AFTER frame 1 had passed. Per derHugo's comment below, I think the best solution is a coroutine as follows:

IEnumerator switchCamera()
{

    yield return null;
    yield return null;
    camManager.requestedFacingDirection = CameraFacingDirection.World;
    
}