For xcoms pushed from different tasks, having the same execution_date and in the same Dag, the result of the xcom_pull with key set as None is not deterministic in its order

**Apache Airflow version**: 2.0


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

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
When tasks in the same dag push Xcoms, the result of the xcom_pull comes in any order. The order is not deterministic.

**What you expected to happen**:

I expected the order to be deterministic.

**How to reproduce it**:

Run the provided example_xcom dag in the repository multiple times


tests/models/test_taskinstance.py
scenario = [State.FAILED, State.SUCCESS, State.FAILED, State.SUCCESS]
        ti_list = self._test_previous_dates_setup(schedule_interval, catchup, scenario)
        self.assertIsNone(ti_list[0].get_previous_execution_date(state=State.SUCCESS))
        self.assertIsNone(ti_list[1].get_previous_execution_date(state=State.SUCCESS))
        self.assertEqual(
            ti_list[3].get_previous_execution_date(state=State.SUCCESS), ti_list[1].execution_date
        )
        self.assertNotEqual(
            ti_list[3].get_previous_execution_date(state=State.SUCCESS), ti_list[2].execution_date
        )
    @parameterized.expand(_prev_dates_param_list)
    def test_previous_start_date_success(self, _, schedule_interval, catchup) -> None:
        scenario = [State.FAILED, State.SUCCESS, State.FAILED, State.SUCCESS]
        ti_list = self._test_previous_dates_setup(schedule_interval, catchup, scenario)
        self.assertIsNone(ti_list[0].get_previous_start_date(state=State.SUCCESS))
        self.assertIsNone(ti_list[1].get_previous_start_date(state=State.SUCCESS))
        self.assertEqual(
            ti_list[3].get_previous_start_date(state=State.SUCCESS),
            ti_list[1].start_date,
        )
        self.assertNotEqual(
            ti_list[3].get_previous_start_date(state=State.SUCCESS),
            ti_list[2].start_date,
        )
    def test_pendulum_template_dates(self):