**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

airflow/jobs/scheduler_job.py
session.query(TI)
                .filter(TI.state != State.SUCCESS, TI.execution_date.in_(sla_dates), TI.dag_id == dag.dag_id)
                .all()
            )
            blocking_tis: List[TI] = []
            for ti in fetched_tis:
                if ti.task_id in dag.task_ids:
                    ti.task = dag.get_task(ti.task_id)
                    blocking_tis.append(ti)
                else:
                    session.delete(ti)
                    session.commit()
            task_list = "\n".join([sla.task_id + ' on ' + sla.execution_date.isoformat() for sla in slas])
            blocking_task_list = "\n".join(
                [ti.task_id + ' on ' + ti.execution_date.isoformat() for ti in blocking_tis]
            )
            
            
            email_sent = False
            notification_sent = False
            if dag.sla_miss_callback:
                
                self.log.info('Calling SLA miss callback')
                try:
                    dag.sla_miss_callback(dag, task_list, blocking_task_list, slas, blocking_tis)
                    notification_sent = True
                except Exception:  
                    self.log.exception("Could not call sla_miss_callback for DAG %s", dag.dag_id)
            email_content = f"""\
            Here's a list of tasks that missed their SLAs: