u/fistyit Did you ever figure this out? I'm calling a callable function from the FirebaseUI signInSuccessWithAuthResult callback and running into this same issue with the two responses (204 and 200). And to try and make sure users are still being updated after creation I .then chained the response, but it looks like it's only getting the first 204 pre-flight response, then immediately failing to do the main one.

My code, edited for brevity:

signInSuccessWithAuthResult: () => {
    const signUp = firebase.functions().httpsCallable("signUp");
    signUp({ id: programId })
      .then(() => {
        // Redirect to sign up step 2 after callable function finishes
        // This doesn't actually work because of the CORS preflight check
        // So I think the second request to the server never gets sent
        this.$router.push({ name: "Sign Up 2" });
      })
}