Getting NotRegistered error on simple task despite [celery] imports configuration

[s-peryt]

I'm encountering a persistent NotRegistered error in my Airflow setup and have exhausted the standard debugging steps. Even a minimal test case fails, suggesting a fundamental issue with how my Celery worker is being configured by Airflow.

My Environment:

• Airflow: apache-airflow==3.0.2
• Providers: apache-airflow-providers-celery==3.12.0
• Executor: CeleryExecutor

Minimal Test Case
To isolate the issue, I removed all other files from my dags folder, leaving only two simple files:

dags/simple_task.py

import logging
from airflow.providers.celery.executors.celery_executor import app

log = logging.getLogger(__name__)

@app.task
def my_simple_test_task(message):
    """A minimal task that only logs a message."""
    log.info("SUCCESS! The simple task ran with message: %s", message)

dags/test_dag.py

from __future__ import annotations
import pendulum
from airflow.decorators import dag, task
from simple_task import my_simple_test_task

@dag(
    dag_id='minimal_celery_test',
    schedule=None,
    start_date=pendulum.now(),
    catchup=False
)
def minimal_celery_test_dag():
    @task
    def trigger_the_simple_task():
        my_simple_test_task.delay("Testing Celery import.")

    trigger_the_simple_task()

minimal_celery_test_dag()

Configuration and Debugging Steps
My airflow.cfg is configured to import this module:

airflow.cfg

[celery]
imports = simple_task

I have already tried the following steps multiple times:

1. Hard Resetting Services: Completely stopping the airflow scheduler and airflow celery worker processes and restarting them.
2. Clearing Cache: Deleting all pycache directories and .pyc files from my project.
3. Verifying File Location: Ensuring both simple_task.py and test_dag.py are directly inside the dags folder which is referenced in config.

The Result
When I run the minimal_celery_test DAG, the trigger_the_simple_task task sends the job, but it immediately fails (as I can see it in Flower dashboard) on the worker with the following error:

NotRegistered('simple_task.my_simple_test_task')

When I check the Celery worker's startup logs, the [tasks] section only lists the default Airflow tasks; my_simple_test_task is missing, which confirms it's not being registered.

My Question:
Given that this minimal configuration appears correct, what could be causing the Airflow Celery worker to completely ignore the [celery] imports setting in airflow.cfg? Are there any other known issues, environmental factors, or configurations specific to Airflow 3 that could lead to this behavior?