Socket.io on App Platform?

Hi, I cannot seem to set up a very simple socket.io/express server on App Platform. Works fine locally, but once I deploy it as a web-server on App Platform, the express server seems to run, but not the socket connection.

Here is the simple index.js file:

const express = require('express');
const app = express();
const http = require('http');
const server = http.createServer(app);
const { Server } = require('socket.io');
const io = new Server(server);

app.get('/', (req, res) => {
  res.sendFile('index.html', { root: __dirname });
});

io.on('connection', (socket) => {
  console.log('a user connected');
});

const PORT = process.env.PORT || 3000;

server.listen(PORT, () => {
  console.log('listening on *:', PORT);
});

When I try to ping it:

curl "<the server URL>/socket.io/?EIO=4&transport=polling"

I get no response.

Any ideas?