In triggered SubDag (schedule_interval=None), when clearing a successful Subdag, child tasks aren't run

[yarongolan]

Apache Airflow version:
Airflow 2.0

Environment:
Ubuntu 20.04 (WSL on Windows 10)

• OS (e.g. from /etc/os-release):
  VERSION="20.04.1 LTS (Focal Fossa)"

• Kernel (e.g. uname -a):
  Linux XXX 4.19.128-microsoft-standard Improving the search functionality in the graph view #1 SMP Tue Jun 23 12:58:10 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

What happened:
After successfully running a SUBDAG, clearing it (including downstream+recursive) doesn't trigger the inner tasks. Instead, the subdag is marked successful and the inner tasks all stay cleared and aren't re-run.

What you expected to happen:
Expected Clear with DownStream + Recursive to re-run all subdag tasks.

How to reproduce it:

1. Using a slightly modified version of https://airflow.apache.org/docs/apache-airflow/stable/concepts.html#subdags:

from airflow import DAG
from airflow.example_dags.subdags.subdag import subdag
from airflow.operators.dummy import DummyOperator
from airflow.operators.subdag import SubDagOperator
from airflow.utils.dates import days_ago

def subdag(parent_dag_name, child_dag_name, args):
    dag_subdag = DAG(
        dag_id=f'{parent_dag_name}.{child_dag_name}',
        default_args=args,
        start_date=days_ago(2),
        schedule_interval=None,
    )

    for i in range(5):
        DummyOperator(
            task_id='{}-task-{}'.format(child_dag_name, i + 1),
            default_args=args,
            dag=dag_subdag,
        )

    return dag_subdag
	
DAG_NAME = 'example_subdag_operator'

args = {
    'owner': 'airflow',
}

dag = DAG(
    dag_id=DAG_NAME, default_args=args, start_date=days_ago(2), schedule_interval=None, tags=['example']
)

start = DummyOperator(
    task_id='start',
    dag=dag,
)

section_1 = SubDagOperator(
    task_id='section-1',
    subdag=subdag(DAG_NAME, 'section-1', args),
    dag=dag,
)

some_other_task = DummyOperator(
    task_id='some-other-task',
    dag=dag,
)

section_2 = SubDagOperator(
    task_id='section-2',
    subdag=subdag(DAG_NAME, 'section-2', args),
    dag=dag,
)

end = DummyOperator(
    task_id='end',
    dag=dag,
)

start >> section_1 >> some_other_task >> section_2 >> end

2. Run the subdag fully.
3. Clear (with recursive/downstream) any of the SubDags.
4. The Subdag will be marked successful, but if you zoom into the subdag, you'll see all the child tasks were not run.

[boring-cyborg]

Thanks for opening your first issue here! Be sure to follow the issue template!

[yarongolan]

Just clarifying that this breaks the SUBDAG functionality completely