@Daniel Thompson mentions that he had forgotten to add {"Content-Type": "application/json"} in the request. He was able to change the request, however, changing requests is not always possible (we are working on the server here).
In my case I needed to force content-type: text/plain to be parsed as json.
If you cannot change the content-type of the request, try using the following code:
app.use(express.json({type: '*/*'}));
Instead of using express.json() globally, I prefer to apply it only where needed, for instance in a POST request:
app.post('/mypost', express.json({type: '*/*'}), (req, res) => {
// echo json
res.json(req.body);
});