Firestore document not deleted

I'm trying to delete a doc from Firestore, I don't get an error but the document is still in the database: simply as that, it keeps on being in the collection it belongs to.

The cloud function is:

exports.countdowns_crud = functions.https.onRequest((request, response) => {

    var req = request;
    var res = response;

    if (request.method === 'DELETE') {
        const countdownId = request.path.split('/')[1];

        const deleteOperation = db.DeleteCountdown(countdownId);

        if (!deleteOperation) {
            console.log('Delete operation result: ', deleteOperation);

            cors(req, res, () => {
                res.status(204).send("DELETED");
            });
        }
        else {
            console.error(addOperation);
            cors(req, res, () => {
                res.status(500).send("INTERNAL SERVER ERROR");
            });
        };

        return;
    }

    cors(req, res, () => {
        res.status(405).send("NOT ALLOWED");
        return;
    });

})

The DeleteCountdown function is in another module:

module.exports = {

  DeleteCountdown: (countdownId) => {
    const countdownsCollection = app.firestore.collection('countdowns');

    countdownsCollection.doc(countdownId).delete()
      .then((res) => {
        console.log('Result: ', res);
        return null;
      })
      .catch((e) => {
        console.error(`unable to delete the countdown ${countdowmnId}: ${e}`);
        return e;
      });
  }

}

This is the logic in a google cloud function, which it's correctly invoked by my react app upon deletion. The passed id is correct, no error is returned, but the doc keeps on living in the collection.