ExpressJS Server Does Not Receive Request Body from Outside React UI

I am working on a simple Node app that involves an ExpressJS server serving up two routes. The routes both need to receive a POSTed JSON body from the request and use that body to perform some functionality. The problem is that I cannot seem to get my setup to work when the request comes from a React UI that is embedded in my company’s website. Initially I was running into CORS errors so I went through configuring and got those cleared. I am now left with the problem that the fetch call coming from my outside React UI is somehow not conveying req.body to my app. This is not a problem when I make the POST request from an HTTP client GUI (i.e. Insomnia or Postman) - req.body is received and an appropriate response is returned. This is only happening in the browser (both Chrome and Firefox) through my unrelated React App (not part of this Digital Ocean Apps deployment).

The fetch call from the React UI is set up like this…

console.log('aiReqObj...', aiReqObj);
let aiResponse = await fetch('https://whateverappname.ondigitalocean.app/preview', {
    method: "POST",
    body: JSON.stringify(aiReqObj)
})

aiReqObj logs out just fine. On my Express server, the route is handled something like this…

app.use(express.json());
app.use(express.urlencoded({ extended: true }));

app.post('/preview', async (req, res) => {
    console.log(req.body);
}

When the request comes from the above fetch call in my react UI, this console.log(req.body) returns an empty object. What may I have overlooked in setting things up? Is this some sort of issue with a CORS header? Any ideas are greatly appreciated.