**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
"""
        Test TaskGroup serialization/deserialization.
        """
        from airflow.operators.dummy import DummyOperator
        from airflow.utils.task_group import TaskGroup
        execution_date = datetime(2020, 1, 1)
        with DAG("test_task_group_serialization", start_date=execution_date) as dag:
            task1 = DummyOperator(task_id="task1")
            with TaskGroup("group234") as group234:
                _ = DummyOperator(task_id="task2")
                with TaskGroup("group34") as group34:
                    _ = DummyOperator(task_id="task3")
                    _ = DummyOperator(task_id="task4")
            task5 = DummyOperator(task_id="task5")
            task1 >> group234
            group34 >> task5
        dag_dict = SerializedDAG.to_dict(dag)
        SerializedDAG.validate_schema(dag_dict)
        json_dag = SerializedDAG.from_json(SerializedDAG.to_json(dag))
        self.validate_deserialized_dag(json_dag, dag)
        serialized_dag = SerializedDAG.deserialize_dag(SerializedDAG.serialize_dag(dag))
        assert serialized_dag.task_group.children
        assert serialized_dag.task_group.children.keys() == dag.task_group.children.keys()
        def check_task_group(node):