Firebase Callable Cloud Function CORS Error

The following is the client side code to call the cloud function:

var getShippingRate = firebase
            .functions()
            .httpsCallable("shippo-generateShippingRate");
          getShippingRate({ address: shippo })
            .then(function(result) {
              // Read result of the Cloud Function.
              console.log("THE SHIPPING RATES", result.data.shipment);
            })
            .catch(function(error) {
              // Getting the Error details.
              console.log("ERROR SHIPPING: ", error);
              var code = error.code;
              var message = error.message;
              var details = error.details;
            });

The cloud function:

exports.generateShippingRate = functions.https.onCall(async (data, context) => {
const customer_address = data.address;

return generateShipmentObject(customer_address);
});

generateshipmentObject returns this:

shippo.shipment.create(
{
  address_from: addressFrom,
  address_to: addressTo,
  parcels: [parcel],
  async: true
},
(err, shipment) => {
  // asynchronously called
  if (err) {
    return { error: err };
  } else {
    return { result: shipment };
  }
}

I get the standard CORS Error, but a callable Cloud Function should handle this automatically:

Access to fetch at ... from origin 'http://localhost:5000' has been blocked by CORS policy:

EDIT

I'm using firebase serve --only hosting to test on localhost.

The Cloud Functions are deployed with firebase deploy --only funtions

I'm calling other similar Cloud Functions on the same site which do not have that issue.