How to await the creation of a document created in cloud functions with onSnapshot

I want to securely create a user document onCreate that is in sync with the auth.user database in Firebase v9. I think it wouldn't be secure to let a registered user create a user document. So I wrote a cloud function which triggers on functions.auth.user().onCreate() and creates a user document.

Problem:

I have the problem keeping them in sync as the onSnapshotmethod which should await for the user document to exists already returns a promise if the user document does not yet exists. Sometimes it works and sometimes not. So I don't know when I can update the by the cloud function created user document.

Question:

Why does the onSnapshot sometimes work and sometimes not. How can I fix it?

Here is a link to a helpful Article which seem to doesn't work in v9. Link

I tried and searched everywhere. I can't believe this is not a standard feature and still a requested topic. This seems so basic.

Error

error FirebaseError: No document to update: as const user = await createAccount(displayName, email, password); returns even if user is not yet in doc.data()

Sign Up function

interface SignUpFormValues {
    email: string;
    password: string;
    confirm: string;
    firstName: string;
    lastName: string;
  }

const createAccount = async (
    displayName: string,
    email: string,
    password: string
  ) => {
    // Create auth user
    const userCredential = await createUserWithEmailAndPassword(
      auth,
      email,
      password
    );
    // -> Signed in

    // Update Profile
    const user = userCredential.user;
    const uid = user.uid;
    await updateProfile(user, {
      displayName: displayName,
    });

    // IMPORTANT: Force refresh regardless of token expiration
    // auth.currentUser.getIdToken(true); // -> will stop the onSnapshot function from resolving properly

    // Build a reference to their per-user document
    const userDocRef = doc(db, "users", uid);

    return new Promise((resolve, reject) => {
      const unsubscribe = onSnapshot(userDocRef, {
        next: (doc) => {
          unsubscribe();
          console.log("doc", doc); // -> returning undefined
          console.log("doc.data()", doc.data()); // -> returning undefined
          resolve(user); // -> returning undefined
        },
        error: (error) => {
          unsubscribe();
          console.log("error", error);
          reject(error);
        },
      });
    });
  };

const handleSignUp = async (values: SignUpFormValues) => {
    const { firstName, lastName, email, password } = values;
    const displayName = `${firstName} ${lastName}`;

    try {
      setError("");
      setLoading(true);

      // Create user account
      const user = await createAccount(displayName, email, password);
      console.log("createAccount -> return:", user); // -> problem here sometimes return undefined

      // Update user
      const newUserData = {
        displayName: displayName,
        firstName,
        lastName,
      };
      // Build a reference to their per-user document
      const userDocRef = doc(db, "users", user.uid);
      await updateDoc(userDocRef, newUserData);

      // Send Email verification
      await authSendEmailVerification(user);

      // Logout
      await logout();

      navigate("/sign-up/email-verification", { state: values });
    } catch (error: any) {
      const errorCode = error.code;
      const errorMessage = error.message;
      console.log("error", error);
      console.log("error", errorCode);
      if (errorCode === "auth/email-already-in-use") {
        const errorMessage =
          "Failed to create an account. E-Mail address is already registered.";
        setError(errorMessage);
        console.log("error", errorMessage);
      } else {
        setError("Failed to create account.");
      }
    }
    setLoading(false);
  };

Cloud function which triggers the user onCreate

// On auth user create
export const authUserWriteListener = functions.auth
  .user()
  .onCreate(async (user, context) => {
    console.log("user:", user);
    const userRef = db.doc(`users/${user.uid}`);
    await userRef.set({
      email: user.email,
      createdAt: context.timestamp,
      firstTimeLogin: true,
    });

    return db.doc("stats/users").update({
      totalDocsCount: FieldValue.increment(1),
    });
  });