State of bool failing to prevent function from repeating

Having trouble tracking this down for some reason, it should be simple but I'm missing something. In PreCountdownTimer() I'm getting two bool properties from GameManager.cs assigning them to _userActive and _preCountdownActive and then looking for changes in Update(). When _userActive = false && _preCountdownActive = false it kicks off StartPreCountTimer() and sets _preCountdownActive to true.

Now once this happens Update() should no longer be able to call StartPreCountTimer() as per the conditional statement...but it still does. This is resulting in my timer getting called over and over again, every frame, thus preventing it from being able to countdown. What do I have wrong here that is preventing the _preCountdownActive bool from stopping Update from firing StartPreCountTimer()?

PreCountdownTimer.cs

using System.Collections;
using UnityEngine;

public class PreCountdownTimer : MonoBehaviour {

    public bool ShowRestartDialog { get; set; }
    private IEnumerator counter;
    private bool _startPrecount;
    private float _preCountdownInterval;
    private bool _preCountdownActive = false;
    private bool _userActive = false;
    private float _timerLength;

    void Start()
    {
        _timerLength = GameManager.Instance.PreCountdownLength;
    }

   void Update()
    {
        _userActive = GameManager.Instance.UserActive;

        if (!_userActive && !_preCountdownActive)
            StartPreCountTimer(_timerLength);
        else if (_userActive && _preCountdownActive)
            StopPreCountTimer();

        Debug.Log("The state of preCountdownActive is: " + _preCountdownActive);
    }

    void StartPreCountTimer(float length)
    {
        _preCountdownActive = true;
        counter = RunTimer(length);
        StartCoroutine(counter);   
    }

    void StopPreCountTimer()
    {
        _preCountdownActive = false;
        StopCoroutine(counter);
    }

    IEnumerator RunTimer(float seconds)
    {
        float s = seconds;
        while (s > 0)
        {
            yield return new WaitForSeconds(_preCountdownInterval);
            s -= _preCountdownInterval;
            Debug.Log("PreCount: " + s);
        }

        if (s == 0)
        {
            _preCountdownActive = false;
            ShowRestartDialog = true;
        }

    }
}

GameManager.cs

using UnityEngine;
using UnityEngine.SceneManagement;

public class GameManager : MonoBehaviour
{
    public static GameManager Instance = null; // create singleton

    public Object introScene;

    public bool UserActive { get; set; }
    public bool OnIntroScreen { get; set; }

    public GameObject preCountdownTimerPrefab;
    private GameObject _preCountdownTimerInstance;
    public float PreCountdownLength { get; protected set; }
    public float PreCountdownInterval { get; protected set; }

    private float _checkMousePositionTimingInterval = 1.0f;
    private Vector3 _currentMousePosition;
    private Vector3 _prevMousePosition;
    private Scene _currentScene;

    void Awake()
    {
        if (Instance == null)
            Instance = this;
        else if (Instance != null)
            Destroy(gameObject);

        DontDestroyOnLoad(gameObject);
    }

    void OnEnable()
    {
        SceneManager.sceneLoaded += OnSceneLoaded;
    }

    void OnSceneLoaded(Scene scene, LoadSceneMode mode)
    {
        _currentScene = scene;
    }

    void Start()
    {
        PreCountdownLength = 5.0f;
        PreCountdownInterval = 1.0f;

        OnIntroScreen = true;
        UserActive = false;

        _prevMousePosition = Input.mousePosition;

        InvokeRepeating("LastMousePosition", 0, _checkMousePositionTimingInterval); 
    }

    void Update()
    {
        _currentMousePosition = Input.mousePosition;

        if (_currentScene.name != introScene.name)
        {
            OnIntroScreen = false;
            if (_currentMousePosition != _prevMousePosition)
                UserActive = true;
            else
                UserActive = false;       
        }
        else if (_currentScene.name == introScene.name)
            OnIntroScreen = true;

        if (!UserActive && !OnIntroScreen)
            if (_preCountdownTimerInstance == null)
                _preCountdownTimerInstance = Instantiate(preCountdownTimerPrefab);
        else if (UserActive)
            if (_preCountdownTimerInstance != null)
                Destroy(_preCountdownTimerInstance);
    }

    void LastMousePosition()
    {
        _prevMousePosition = Input.mousePosition;
    }
}