**Apache Airflow version**: 2.0.0


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

**Environment**: 

- **Cloud provider or hardware configuration**: local/aws
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04.5 LTS
- **Kernel** (e.g. `uname -a`): 5.4.0-1032-aws
- **Install tools**: pip
- **Others**:

**What happened**:
I did a fresh Airflow 2.0.0 install. With this version, when I manually trigger a DAG, Airflow skips the next scheduled run.
<!-- (please include exact error messages if you can) -->

**What you expected to happen**:
Manual runs do not interfere with the scheduled runs prior to Airflow 2. 
<!-- What do you think went wrong? -->

**How to reproduce it**:
Create a simple hourly DAG. After enabling it and the initial run, run it manually. It shall skip the next hour. Below is an example, where the manual run with execution time of 08:17 causes the scheduled run with execution time of 08:00 to skip. 
![image](https://user-images.githubusercontent.com/26160471/103457719-c7193480-4d12-11eb-82cb-42efaedc9ef4.png)


tests/jobs/test_scheduler_job.py
dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        dag_file_processor.manage_slas(dag=dag, session=session)
        sla_callback.assert_not_called()
    def test_dag_file_processor_sla_miss_callback_exception(self):
        """
        Test that the dag file processor gracefully logs an exception if there is a problem
        calling the sla_miss_callback
        """
        session = settings.Session()
        sla_callback = MagicMock(side_effect=RuntimeError('Could not call function'))
        test_start_date = days_ago(2)
        dag = DAG(
            dag_id='test_sla_miss',
            sla_miss_callback=sla_callback,
            default_args={'start_date': test_start_date},
        )
        task = DummyOperator(task_id='dummy', dag=dag, owner='airflow', sla=datetime.timedelta(hours=1))
        session.merge(TaskInstance(task=task, execution_date=test_start_date, state='Success'))
        
        session.merge(SlaMiss(task_id='dummy', dag_id='test_sla_miss', execution_date=test_start_date))
        
        mock_log = mock.MagicMock()
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock_log)
        dag_file_processor.manage_slas(dag=dag, session=session)
        assert sla_callback.called
        mock_log.exception.assert_called_once_with(
            'Could not call sla_miss_callback for DAG %s', 'test_sla_miss'
        )
    @mock.patch('airflow.jobs.scheduler_job.send_email')
    def test_dag_file_processor_only_collect_emails_from_sla_missed_tasks(self, mock_send_email):