**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
cls,
        encoded_group: Dict[str, Any],
        parent_group: Optional[TaskGroup],
        task_dict: Dict[str, BaseOperator],
    ) -> Optional[TaskGroup]:
        """Deserializes a TaskGroup from a JSON object."""
        if not encoded_group:
            return None
        group_id = cls._deserialize(encoded_group["_group_id"])
        kwargs = {
            key: cls._deserialize(encoded_group[key])
            for key in ["prefix_group_id", "tooltip", "ui_color", "ui_fgcolor"]
        }
        group = SerializedTaskGroup(group_id=group_id, parent_group=parent_group, **kwargs)
        group.children = {
            label: task_dict[val]
            if _type == DAT.OP  
            else SerializedTaskGroup.deserialize_task_group(val, group, task_dict)
            for label, (_type, val) in encoded_group["children"].items()
        }
        group.upstream_group_ids = set(cls._deserialize(encoded_group["upstream_group_ids"]))
        group.downstream_group_ids = set(cls._deserialize(encoded_group["downstream_group_ids"]))
        group.upstream_task_ids = set(cls._deserialize(encoded_group["upstream_task_ids"]))
        group.downstream_task_ids = set(cls._deserialize(encoded_group["downstream_task_ids"]))
        return group