**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/serialization/serialized_objects.py
"""A JSON serializable representation of TaskGroup."""
    @classmethod
    def serialize_task_group(cls, task_group: TaskGroup) -> Optional[Union[Dict[str, Any]]]:
        """Serializes TaskGroup into a JSON object."""
        if not task_group:
            return None
        serialize_group = {
            "_group_id": task_group._group_id,  
            "prefix_group_id": task_group.prefix_group_id,
            "tooltip": task_group.tooltip,
            "ui_color": task_group.ui_color,
            "ui_fgcolor": task_group.ui_fgcolor,
            "children": {
                label: (DAT.OP, child.task_id)
                if isinstance(child, BaseOperator)
                else (DAT.TASK_GROUP, SerializedTaskGroup.serialize_task_group(child))
                for label, child in task_group.children.items()
            },
            "upstream_group_ids": cls._serialize(list(task_group.upstream_group_ids)),
            "downstream_group_ids": cls._serialize(list(task_group.downstream_group_ids)),
            "upstream_task_ids": cls._serialize(list(task_group.upstream_task_ids)),
            "downstream_task_ids": cls._serialize(list(task_group.downstream_task_ids)),
        }
        return serialize_group
    @classmethod
    def deserialize_task_group(