Freezetime does not work with FastAPI test client

Freezetime does not seem to work with FastAPI TestClient. I have build this simple example, the test is failing. Freezetime does not override datetime in this case :/

import datetime

from fastapi import FastAPI
from fastapi.testclient import TestClient
from pydantic import BaseModel
from freezegun import freeze_time

app = FastAPI()


class Message(BaseModel):
    message: str = "Hello World"
    timestamp: datetime.datetime = datetime.datetime.utcnow()


@app.get("/", response_model=Message)
def main() -> Message:
    return Message()


client = TestClient(app)

@freeze_time('2022-09-18T13:36:41.624237')
def test_read_main():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {
            'message': 'Hello World',
            'timestamp': '2022-09-18T13:36:41.624237'
            }

when I run pytest I am getting this message

    @freeze_time('2022-09-18T13:36:41.624237')
    def test_read_main():
        response = client.get("/")
        assert response.status_code == 200
>       assert response.json() == {
                'message': 'Hello World',
                'timestamp': '2022-09-18T13:36:41.624237'
                }
E       AssertionError: assert {'message': '...44:25.021208'} == {'message': '...36:41.624237'}
E         Omitting 1 identical items, use -vv to show
E         Differing items:
E         {'timestamp': '2022-09-18T13:44:25.021208'} != {'timestamp': '2022-09-18T13:36:41.624237'}
E         Use -v to get more diff

Any ideas if these kind of tests are possible with the FastAPI TestClient ?