Why yield return null can't executed in Start() void

This problem is not always occurred.But when it happened,I can't find a way to fix it. Here is the code:

    void Start()
    {
        StartCoroutine(test());
    }

    IEnumerator test()
    {
        LoadingBar.maxValue = 1.0f;
        //This is the original code.
        //foreach (var v in Loader)
        //{
        //    Debug.Log(v.Key);
        //    yield return v.Value.Loading();
        //    LoadingBar.value = TotalLoadingProgress;
        //}
        //This is the simplified code.
        for (int i = 0; i < 9; i++)
        {
            Debug.Log("a");//This will show
            yield return null;
            Debug.Log("b");//This will not
        }

        OnLoadingCompleted?.Invoke();
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.Return))
            StartCoroutine(test());
    }

There is a "yield return null" in every manager's Loading() method.Sometimes it work as i expect,all 9 manager can successfully loaded.But sometimes it's just stuck at "yield return null".

I simplified the code like above.When this test coroutine get started in Start() void,it will only debug one single message:"a" But when this coroutine get started in Update(),all 9 "a" and "b" will showing up as expect.

I have no idea why this code is normal in update but get abnormal sometimes in start void.

learn from my lessons! I writed code like this to switch the UI:

private void SwitchUI()
{
    LoadingMenu.gameObject.SetActive(false);
    MainMenuUI.gameObject.SetActive(false);
    BattleUI.gameObject.SetActive(false);
    switch (SceneController.Instance.CurrentMap)
    {
        case "LoaderScene":
            LoadingMenu.gameObject.SetActive(true);
            break;
        case "MainMenu":
            MainMenuUI.gameObject.SetActive(true);
            break;
        default:
            BattleUI.gameObject.SetActive(true);
            break;
    }
}

And this code caused my problem.The instant SetActive false and setback to true will still break the coroutine.I have to deactive all object at first in editor mode,after that i can enter play mode.