Requests not being forwarded by proxy to backend

[flixman]

I am working on a toy project to learn about react, and I have it structured like

project
├── app.py
├── backend
└── frontend
       └── src
           ├── App.tsx
           └── setupProxy.js

The frontend is in react (non ejected) and the backend is in python, listening to port 5000. I have added to /etc/hosts the names that are required for the project to work (lists.local for the main app, redis.local, etc.) so that I can get a local development environment reasonably closer to what will be found in production. This setup runs correctly whenever I build the react app and go for everything production like. However, whenever I run "npm run start", the requests are not forwarded to the backend.

Contents of setupProxy.js (I only need to forward anything below /api):

const { createProxyMiddleware } = require('http-proxy-middleware');

module.exports = (app) => {
    app.use(
        '/api',
        createProxyMiddleware({
            target: 'http://lists.local:5000',
            changeOrigin: true
        })
    );
};

In package.json I have the following entry:

{
    ....
    "scripts": {
        "start": "HOST=lists.local react-scripts start",
        ....
    },
    ....
}

What am I doing wrong?

EDIT: This is getting more and more weird: if I copy (but not if I move) the file setupProxy.js to the frontend folder (so that the same file is found in frontend and in frontend/src) the requests are forwarded. Otherwise, nothing. This feels wrong....