**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
ti = dr.get_task_instance('test_task')
        ti.set_state(state, session)
        scheduler._schedule_dag_run(dr, set(), session)
        
        scheduler._send_dag_callbacks_to_processor.assert_called_once_with(dr, None)
        session.rollback()
        session.close()
    def test_do_not_schedule_removed_task(self):
        dag = DAG(dag_id='test_scheduler_do_not_schedule_removed_task', start_date=DEFAULT_DATE)
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        session = settings.Session()
        dag.sync_to_db(session=session)
        session.flush()
        dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
        dr = dag.create_dagrun(
            execution_date=DEFAULT_DATE,
            run_type=DagRunType.SCHEDULED,
            state=State.RUNNING,
            session=session,
        )
        assert dr is not None
        
        dag = DAG(dag_id='test_scheduler_do_not_schedule_removed_task', start_date=DEFAULT_DATE)
        scheduler = SchedulerJob(subdir=os.devnull)
        res = scheduler._executable_task_instances_to_queued(max_tis=32, session=session)
        assert [] == res
        session.rollback()
        session.close()
    @provide_session
    def evaluate_dagrun(