[Unity]Unity's coroutine can't be stopped if the IEnumerator call stack is too deep

Unity's coroutine can't be stopped if the IEnumerator call stack is too deep

In the following example, if  Co3() is executed, then the coroutine can't be stopped by StopCoroutine()

public class TestCoroutine : MonoBehaviour {
    private Coroutine coroutine;
 // Use this for initialization
 void Start () {
        coroutine = StartCoroutine(Co1());
 }
 
 // Update is called once per frame
 void Update () {
  if(Input.GetKeyDown(KeyCode.S))
        {
            if(coroutine != null)
            {
                StopCoroutine(coroutine);
                Debug.LogError("Stop coroutine");
            }
        }
 }

    private IEnumerator Co1()
    {
        Debug.LogError("Co1 start");

        yield return new WaitForSeconds(5);
        Debug.LogError("Co1 end");
        yield return Co2();

    }

    private IEnumerator Co2()
    {
        Debug.LogError("Co2 start");
        float time = Time.time;
        while(Time.time - time < 5)
        {
            yield return null;
        }
        Debug.LogError("Co2 end");
        yield return Co3();
    }

    private IEnumerator Co3()
    {
        //coroutine can't be stopped by StopCoroutine if here is executed
        Debug.LogError("Co3 start");
        float time = Time.time;
        while (Time.time - time < 5)
        {
            yield return null;
        }
        Debug.LogError("Co3 end");

    }
}