title: Airflow ExternalTaskSensor poking another dag all the time
body: <p>I want to use ExternalTaskSensor。</p>
  
  <pre><code>TaskSensor = ExternalTaskSensor(
                              task_id='TaskSensor',
                              external_dag_id='datamart_OTT_CMS_v1',
                               external_task_id="end",
                               allowed_states=None,
                               execution_delta=None,
                               execution_date_fn=None,
                                  dag=dag)
  </code></pre>
  
  <p>I write a dag B depend on A. When dag A end (successed) then dag B will run. But when it's running, I found that dag B is poking dag A all the time.
  <a href="https://i.stack.imgur.com/Oqz0P.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Oqz0P.png" alt="enter image description here"></a> </p>
  
  <p><a href="https://i.stack.imgur.com/B0Wnd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/B0Wnd.png" alt="enter image description here"></a></p>
  
  <p>why? am I miss something?</p>