**Apache Airflow version**:

master

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**: breeze

**What happened**:

This DAG
```python
from airflow import models
from airflow.operators.dummy_operator import DummyOperator
from airflow.utils.dates import days_ago

with models.DAG("test", start_date=days_ago(1), schedule_interval=None,) as dag:
    t1 = DummyOperator(task_id="t1")
    t2 = DummyOperator(task_id="t2")
    t1 >> t2
```

is rendering like that:

<img width="1374" alt="Screenshot 2020-08-27 at 19 59 41" src="https://user-images.githubusercontent.com/9528307/91478403-11d7fb00-e8a0-11ea-91d0-d7d578bcb5a2.png">

**What you expected to happen**:

I expect to see same relations as defined in DAG file

**How to reproduce it**:

Render the example DAG from above.


**Anything else we need to know**:

I'm surprised by this bug 👀 

airflow/www/views.py
"""Clears the action."""
        try:
            dag_to_tis = {}
            for ti in task_instances:
                dag = current_app.dag_bag.get_dag(ti.dag_id)
                task_instances_to_clean = dag_to_tis.setdefault(dag, [])
                task_instances_to_clean.append(ti)
            for dag, task_instances_list in dag_to_tis.items():
                models.clear_task_instances(task_instances_list, session, dag=dag)
            session.commit()
            flash("{0} task instances have been cleared".format(len(task_instances)))
            self.update_redirect()
            return redirect(self.get_redirect())
        except Exception:  
            flash('Failed to clear task instances', 'error')
    @provide_session
    def set_task_instance_state(self, tis, target_state, session=None):