Flask Session not storing data correctly

Here's my front-end that's sending the data:

async function notifyBackendAboutUpload(fileName) {    try {        const response = await fetch('http://127.0.0.1:5000/notify-upload', {            method: 'POST',            headers: {'Content-Type': 'application/json',            },            body: JSON.stringify({ fileName }),            credentials: 'include',        });        const responseData = await response.json();        console.log(responseData);    } catch (error) {        console.error('Error notifying the backend:', error);    }

Here's my __init__.py:

from flask import Flask, sessionfrom flask_cors import CORSfrom dotenv import load_dotenvimport osdef create_app():    load_dotenv()  # Load environment variables    app = Flask(__name__)    # Configuration from environment variables    app.config["AWS_ACCESS_KEY_ID"] = os.getenv("AWS_ACCESS_KEY_ID")    app.config["AWS_SECRET_ACCESS_KEY"] = os.getenv("AWS_SECRET_ACCESS_KEY")    app.config["AWS_REGION"] = os.getenv("AWS_REGION")    app.config["SECRET_KEY"] = os.getenv("SECRET_KEY")    app.config["SESSION_COOKIE_SAMESITE"] = "None"    CORS(app, supports_credentials=True)    from app.routes.routes import routes as routes_blueprint    app.register_blueprint(routes_blueprint)    # Potentially add more blueprint registrations here    return app

Here's my routes.py:

from flask import Blueprint, jsonify, request, session, redirect, url_forfrom flask_cors import CORSfrom app.services.aws_service import fetch_csv_from_s3routes = Blueprint("routes", __name__)CORS(routes, supports_credentials=True)@routes.route("/test-s3-fetch")def test_s3_fetch():    bucket_name = "instanalytics"    object_key = session.get("fileName")    print('Retrieved from session:', object_key)    if not object_key:        return jsonify({'error': 'No file name provided'}), 400    # df = fetch_csv_from_s3(bucket_name, object_key)    return jsonify({"message": f"FileName from session: {object_key}"})    # return jsonify(df.head().to_dict())@routes.route('/notify-upload', methods=['POST'])def notify_upload():    data=request.get_json()    fileName=data['fileName']    if not fileName:        return jsonify({'error': 'Filename is required'}), 400    session['fileName'] = fileName    print('Stored in session:', session.get('fileName'))    return redirect(url_for("routes.test_s3_fetch"))@routes.route("/test-session")def test_session():    # Directly manipulate and check session data    session["test"] = "Session is working!"    print("Session data:", session)    return jsonify({"testSessionValue": session.get("test")})

Now, I'm trying to use the sent file name from the front-end to use here and it's storing the fileName fine but it's not retrieving it correctly for some reason. Here's the logs:

Stored in session: aggregate.csv127.0.0.1 - - [04/Apr/2024 05:47:18] "POST /notify-upload HTTP/1.1" 302 -Retrieved from session: None127.0.0.1 - - [04/Apr/2024 05:47:18] "GET /test-s3-fetch HTTP/1.1" 400 -

I'm new to flask so please let me know what I'm doing wrong. Thanks.