**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
session.add(TaskInstance(task1, DEFAULT_DATE, State.REMOVED))
        session.flush()
        run1 = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=DEFAULT_DATE + timedelta(hours=1),
            state=State.RUNNING,
            session=session,
        )
        dag.sync_to_db(session=session)  
        job = SchedulerJob(subdir=os.devnull)
        job.executor = MockExecutor(do_update=False)
        job.processor_agent = mock.MagicMock(spec=DagFileProcessorAgent)
        num_queued = job._do_scheduling(session)
        assert num_queued == 1
        ti = run1.get_task_instance(task1.task_id, session)
        assert ti.state == State.QUEUED
    def test_do_schedule_max_active_runs_and_manual_trigger(self):
        """
        Make sure that when a DAG is already at max_active_runs, that manually triggering a run doesn't cause
        the dag to "stall".
        """
        with DAG(
            dag_id='test_max_active_run_plus_manual_trigger',
            start_date=DEFAULT_DATE,
            schedule_interval='@once',
            max_active_runs=1,
        ) as dag:
            
            task1 = BashOperator(task_id='dummy1', bash_command='true')
            task2 = BashOperator(task_id='dummy2', bash_command='true')