"Fetch failed loading" - fetch works in Postman but fails in Chrome & Safari

Update: Fixed. It looks like the request was coming back as a 503 (as it should), then my app refreshed and displayed the non-error message: "Fetch failed loading". I just wasn't seeing the response because of the refresh.

I am not able to make a fetch request from my locally-hosted Create-React-App to my Heroku-hosted Node server.

My Node server has CORS enabled. If I make a POST request via Postman, I get an appropriate response (503, because currently there is no database hooked up, so the data is not being saved. If I should be sending back a different response, let me know). My Postman request has 'application/json' as the content-type, no authorization, and a body of { "rating": "5", "zipcode": "0" }.

However, when I make a POST request from my React app, I get a message in my console: "Fetch failed loading: OPTIONS "https://shielded-gorge-69158.herokuapp.com/feedback"." There is no associated error, only the message. There is no information about the request in my Network panel.

The fetch request works when I do it locally, from localhost:3000 (my app) to localhost:5000 (my server). It only fails when I try to make the request to the (otherwise identical) server hosted on Heroku.

This is what the fetch request looks like:

return fetch('https://shielded-gorge-69158.herokuapp.com/feedback', {
  method: 'POST',
  headers: {
    'Content-type': 'application/json'
  },
  body: JSON.stringify({ rating: userRating, zipcode: userZip })
}).then(res => {
if (!res.ok) {
  throw new Error('Error:', res.statusText);
}
  return res;
}).catch(err => console.error(err));

Edit: I'm continuing to research and it seems like Postman shouldn't/doesn't make preflight requests (https://github.com/postmanlabs/postman-app-support/issues/2845). So perhaps that is the issue — but why would the request be working when my server is local, but not when it is hosted on Heroku?