**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
"""Mark DagRun success"""
        dag_id = request.form.get('dag_id')
        execution_date = request.form.get('execution_date')
        confirmed = request.form.get('confirmed') == 'true'
        origin = get_safe_url(request.form.get('origin'))
        return self._mark_dagrun_state_as_success(dag_id, execution_date,
                                                  confirmed, origin)
    def _mark_task_instance_state(self,  
                                  dag_id, task_id, origin, execution_date,
                                  confirmed, upstream, downstream,
                                  future, past, state):
        dag = current_app.dag_bag.get_dag(dag_id)
        task = dag.get_task(task_id)
        task.dag = dag
        latest_execution_date = dag.get_latest_execution_date()
        if not latest_execution_date:
            flash(f"Cannot make {state}, seem that dag {dag_id} has never run", "error")
            return redirect(origin)
        execution_date = timezone.parse(execution_date)
        from airflow.api.common.experimental.mark_tasks import set_state
        if confirmed:
            altered = set_state(tasks=[task], execution_date=execution_date,
                                upstream=upstream, downstream=downstream,
                                future=future, past=past, state=state,
                                commit=True)
            flash("Marked {} on {} task instances".format(state, len(altered)))
            return redirect(origin)
        to_be_altered = set_state(tasks=[task], execution_date=execution_date,
                                  upstream=upstream, downstream=downstream,
                                  future=future, past=past, state=state,