Setting up unit testing firebase functions (online mode).

I'm testing an onCreate() function, so I need to create a document in firestore to ensure the function is triggered and working properly. The problem I'm encountering is that testEnv.firestore.makeDocumentSnapshot(data, path) is not creating a new document. If the document already exists, I can write this data to it and trigger the onCreate() function, but if it doesn't exist I get an Error: 5 NOT_FOUND: No document to update when I run the test.

test.ts

const functions = require("firebase-functions-test");

const testEnv = functions({
  databaseURL: "https://***.firebaseio.com",
  storageBucket: "***.appspot.com",
  projectId: "***",
}, "./test-service-account.json");

import "jest";
import * as admin from "firebase-admin";


import { makeLowerCase } from "../src";

describe("makes bio lower case", () => {
  let wrapped: any;
  beforeAll(() => {
    wrapped = testEnv.wrap(makeLowerCase);
  });

  test("it converts the bio to lowercase", async () => {

    const path = "/animals/giraffe";
    const data = {bio: "GIRAFFE"};

    const snap = testEnv.firestore.makeDocumentSnapshot(data, path);

    await wrapped(snap)

    const after = await admin.firestore().doc(path).get();

    expect(after?.data()?.bio).toBe("giraffe");

  });
});

makeLowerCase.ts

import * as functions from "firebase-functions";
import * as admin from "firebase-admin";

export const makeLowerCase = functions.firestore
  .document("animals/{animalId}")
  .onCreate((snap, context) => {
    const data = snap.data();
    const bio = data.bio.toLowerCase();

    return admin.firestore().doc(`animals/${snap.id}`).update({bio});
  });

I can fix this in makeLowerCase.ts by returning:

admin.firestore().doc(`animals/${snap.id}`).set({bio}, {merge: true});

Or by creating the doc with admin in the test:

await admin.firestore().doc(path).set(data);

But the testEnv.firestore.makeDocumentSnapshot(data, path); should be creating a document I think, no?

Is this is bug or expected behaviour from firebase-functions-test"?