**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
return list(self.task_dict.values())
    @tasks.setter
    def tasks(self, val):
        raise AttributeError('DAG.tasks can not be modified. Use dag.add_task() instead.')
    @property
    def task_ids(self) -> List[str]:
        return list(self.task_dict.keys())
    @property
    def task_group(self) -> "TaskGroup":
        return self._task_group
    @property
    def filepath(self) -> str:
        """File location of where the dag object is instantiated"""
        fn = self.full_filepath.replace(settings.DAGS_FOLDER + '/', '')
        fn = fn.replace(os.path.dirname(__file__) + '/', '')
        return fn
    @property
    def folder(self) -> str:
        """Folder location of where the DAG object is instantiated."""
        return os.path.dirname(self.full_filepath)
    @property
    def owner(self) -> str:
        """
        Return list of all owners found in DAG tasks.
        :return: Comma separated list of owners in DAG tasks
        :rtype: str
        """
        return ", ".join({t.owner for t in self.tasks})
    @property
    def allow_future_exec_dates(self) -> bool: