**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
dag = dagbag.dags[request.dag_id]
        dag_run = dag.get_dagrun(execution_date=request.execution_date, session=session)
        dag.handle_callback(
            dagrun=dag_run, success=not request.is_failure_callback, reason=request.msg, session=session
        )
    def _execute_task_callbacks(self, dagbag: DagBag, request: TaskCallbackRequest):
        simple_ti = request.simple_task_instance
        if simple_ti.dag_id in dagbag.dags:
            dag = dagbag.dags[simple_ti.dag_id]
            if simple_ti.task_id in dag.task_ids:
                task = dag.get_task(simple_ti.task_id)
                ti = TI(task, simple_ti.execution_date)
                
                ti.start_date = simple_ti.start_date
                ti.end_date = simple_ti.end_date
                ti.try_number = simple_ti.try_number
                ti.state = simple_ti.state
                ti.test_mode = self.UNIT_TEST_MODE
                if request.is_failure_callback:
                    ti.handle_failure_with_callback(error=request.msg, test_mode=ti.test_mode)
                    self.log.info('Executed failure callback for %s in state %s', ti, ti.state)
    @provide_session
    def process_file(