**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
num_active_runs.get(dag.dag_id, 0),
            )
            for orm_tag in list(orm_dag.tags):
                if orm_tag.name not in orm_dag.tags:
                    session.delete(orm_tag)
                orm_dag.tags.remove(orm_tag)
            if dag.tags:
                orm_tag_names = [t.name for t in orm_dag.tags]
                for dag_tag in list(dag.tags):
                    if dag_tag not in orm_tag_names:
                        dag_tag_orm = DagTag(name=dag_tag, dag_id=dag.dag_id)
                        orm_dag.tags.append(dag_tag_orm)
                        session.add(dag_tag_orm)
        if settings.STORE_DAG_CODE:
            DagCode.bulk_sync_to_db([dag.fileloc for dag in orm_dags])
        
        
        session.flush()
        for dag in dags:
            cls.bulk_write_to_db(dag.subdags, session=session)
    @provide_session
    def sync_to_db(self, session=None):
        """
        Save attributes about this DAG to the DB. Note that this method
        can be called for both DAGs and SubDAGs. A SubDag is actually a
        SubDagOperator.
        :return: None
        """
        self.bulk_write_to_db([self], session)
    def get_default_view(self):