**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)


airflow/models/dag.py
if default_view in DEFAULT_VIEW_PRESETS:
            self._default_view: str = default_view
        else:
            raise AirflowException(
                f'Invalid values of dag.default_view: only support '
                f'{DEFAULT_VIEW_PRESETS}, but get {default_view}'
            )
        if orientation in ORIENTATION_PRESETS:
            self.orientation = orientation
        else:
            raise AirflowException(
                f'Invalid values of dag.orientation: only support '
                f'{ORIENTATION_PRESETS}, but get {orientation}'
            )
        self.catchup = catchup
        self.is_subdag = False  
        self.partial = False
        self.on_success_callback = on_success_callback
        self.on_failure_callback = on_failure_callback
        
        
        self.has_on_success_callback = self.on_success_callback is not None
        self.has_on_failure_callback = self.on_failure_callback is not None
        self.doc_md = doc_md
        self._access_control = DAG._upgrade_outdated_dag_access_control(access_control)
        self.is_paused_upon_creation = is_paused_upon_creation
        self.jinja_environment_kwargs = jinja_environment_kwargs
        self.tags = tags
        self._task_group = TaskGroup.create_root(self)
    def __repr__(self):