**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
ti1.state = State.SUCCESS
        ti3 = dag_run.get_task_instance(task3.task_id, session)
        ti3.state = State.RUNNING
        session.flush()
        
        num_queued = job._do_scheduling(session)
        assert num_queued == 1
        
        ti2 = dag_run.get_task_instance(task2.task_id, session)
        assert ti2.state == State.QUEUED
        ti2.state = None
        session.flush()
        
        manual_run = dag.create_dagrun(
            run_type=DagRunType.MANUAL,
            execution_date=DEFAULT_DATE + timedelta(hours=1),
            state=State.RUNNING,
            session=session,
        )
        session.flush()
        num_queued = job._do_scheduling(session)
        assert num_queued == 1
        
        ti2 = dag_run.get_task_instance(task2.task_id, session)
        assert ti2.state == State.QUEUED
        
        ti1 = manual_run.get_task_instance(task1.task_id, session)
        assert ti1.state is None
@pytest.mark.xfail(reason="Work out where this goes")
def test_task_with_upstream_skip_process_task_instances():