**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
dr2 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=dag.following_schedule(dr1.execution_date),
            state=State.RUNNING,
        )
        ti3 = TaskInstance(task1, dr2.execution_date)
        ti4 = TaskInstance(task2, dr2.execution_date)
        ti3.refresh_from_db()
        ti4.refresh_from_db()
        
        ti3.state = State.SCHEDULED
        ti4.state = State.SCHEDULED
        session.merge(ti3)
        session.merge(ti4)
        session.flush()
        assert State.RUNNING == dr2.state
        res = scheduler._critical_section_execute_task_instances(session)
        
        ti1.refresh_from_db()
        ti2.refresh_from_db()
        ti3.refresh_from_db()
        ti4.refresh_from_db()
        assert 3 == DAG.get_num_task_instances(
            dag_id, dag.task_ids, states=[State.RUNNING, State.QUEUED], session=session
        )
        assert State.RUNNING == ti1.state
        assert State.RUNNING == ti2.state
        assert {State.QUEUED, State.SCHEDULED} == {ti3.state, ti4.state}
        assert 1 == res
    def test_execute_task_instances_limit(self):