**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/serialization/test_dag_serialization.py
"__var": {
                "depends_on_past": False,
                "retries": 1,
                "retry_delay": {"__type": "timedelta", "__var": 300.0},
            },
        },
        "start_date": 1564617600.0,
        '_task_group': {
            '_group_id': None,
            'prefix_group_id': True,
            'children': {'bash_task': ('operator', 'bash_task'), 'custom_task': ('operator', 'custom_task')},
            'tooltip': '',
            'ui_color': 'CornflowerBlue',
            'ui_fgcolor': '#000',
            'upstream_group_ids': [],
            'downstream_group_ids': [],
            'upstream_task_ids': [],
            'downstream_task_ids': [],
        },
        "is_paused_upon_creation": False,
        "_dag_id": "simple_dag",
        "fileloc": None,
        "tasks": [
            {
                "task_id": "bash_task",
                "owner": "airflow",
                "retries": 1,
                "retry_delay": 300.0,
                "_downstream_task_ids": [],
                "_inlets": [],