Apache Airflow version: 2.0.0

What happened:

Sensor start_date gets updated to each time the task instance is rescheduled for the same try.

What you expected to happen:

I expect the start_date of a sensor task instance to remain the same within a given try. The start_date should be the start of first poke instead of the start time of the following rescheduled attempts.

This behaviour does not match the description in the source code.

        # Set the task start date. In case it was re-scheduled use the initial
        # start date that is recorded in task_reschedule table

Airflow did behave as expected in 1.10.x but it stopped in 2.0+.

How to reproduce it:

Run any sensors and watch the start_date value change at after each retry.

with DAG(
        dag_id="check_dag",
        start_date=datetime(2021, 1, 1),
        schedule_interval='@daily',
        catchup=False,
) as dag:
    p = ExternalTaskSensor(
        task_id='sensor_task',
        external_dag_id='my_dag',
        mode='reschedule'
    )