Problem "yield break" and "weakreference"

"yield break;" has a problem;

example

 IEnumerator A()
 {
     Debug.Log("A Start");
     yield break;
     Debug.Log"A End");
 }
 
 IEnumerator B()
 {
     Debug.Log("B Start");
     yield StartCoroutine(A());
     Debug.Log("B End");
 }
 
 public Class Test : Monobehivor
 {
 
    public WeakReference pt;
 
    void Start()
    {
        pt = new System.WeakReference(StartCoroutine(B()));
    }
 
    void Update()
   {
       if (pt.Target == null)
      {
           Debug.Log("Reset");
      }
   }
  
 }

that result is ...

"B Start" "A Start" "Reset" "B End"

the problem is that the weakreference of B function is reset on the "yield StartCoroutine(A());" (pt.Target is null) and the "Debug.Log("B End") is call ...