**Apache Airflow version**: 2.0.0


**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**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

tests/jobs/test_scheduler_job.py
execution_date = DEFAULT_DATE
        dag_id = "dag_id"
        task_id = "task_id"
        try_number = 42
        executor = MagicMock()
        scheduler = SchedulerJob(executor=executor)
        scheduler.processor_agent = MagicMock()
        event_buffer = {TaskInstanceKey(dag_id, task_id, execution_date, try_number): (State.SUCCESS, None)}
        executor.get_event_buffer.return_value = event_buffer
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE)
        task = DummyOperator(dag=dag, task_id=task_id)
        with create_session() as session:
            ti = TaskInstance(task, DEFAULT_DATE)
            ti.state = State.SUCCESS
            session.merge(ti)
        scheduler._process_executor_events()
        
        
        assert event_buffer == {}
    def test_execute_task_instances_is_paused_wont_execute(self):