**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
#
#
"""Serialized DAG and BaseOperator"""
import datetime
import enum
import logging
from inspect import Parameter, signature
from typing import TYPE_CHECKING, Any, Dict, Iterable, List, Optional, Set, Union
import cattr
import pendulum
from dateutil import relativedelta
try:
    from functools import cache
except ImportError:
    from functools import lru_cache
    cache = lru_cache(maxsize=None)
from pendulum.tz.timezone import Timezone
from airflow.exceptions import AirflowException, SerializationError
from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
from airflow.models.connection import Connection
from airflow.models.dag import DAG
from airflow.providers_manager import ProvidersManager
from airflow.serialization.enums import DagAttributeTypes as DAT, Encoding
from airflow.serialization.helpers import serialize_template_field
from airflow.serialization.json_schema import Validator, load_dag_schema
from airflow.settings import json
from airflow.utils.code_utils import get_python_source