Can't get React + socket.io to work on Heroku - node.js

Question

I'm trying to set up a simple socket.io echo server on Heroku using Express and React. The server returns the React site from the build folder, then listens for incoming messages using onAny(). Everything works fine locally, but when deployed to Heroku none of the client-emitted messages are going through after the connection has been established.
I've used Heroku's guide as well as create-react-app and this Medium article as my starting points, and did make sure to turn on http-session-affinity as the Heroku guide said to.
Currently, the React client is set up to emit a fixed message through the onclick handler of a <span>.
Here's the contents of server.js:
'use strict';
const path = require('path');
const express = require('express');
const socketIO = require('socket.io');
const PORT = process.env.PORT || 3000;
const app = express()
.use(express.static('build'))
.listen(PORT, () => console.log(`Listening on ${PORT}`));
const io = socketIO(app);
io.on('connection', (socket) => {
console.log('Client connected');
socket.on('disconnect', () => console.log('Client disconnected'));
socket.onAny((type, data) => {
console.log('Received:');
console.log(data);
socket.emit('echo', data);
});
});
And here is the client-side React hook that establishes the connection and emits the messages, where I replace [app-name] with the Heroku app name:
import { useEffect, useRef, useState } from "react";
import socketIOClient from "socket.io-client";
const NEW_CHAT_MESSAGE_EVENT = "newChatMessage";
// const SOCKET_SERVER_URL = 'localhost:3000';
const SOCKET_SERVER_URL = 'wss://[app-name].herokuapp.com/sockjs-node';
const useChat = (roomId) => {
const [messages, setMessages] = useState([]);
const socketRef = useRef();
useEffect(() => {
socketRef.current = socketIOClient(SOCKET_SERVER_URL, {
query: { roomId },
});
socketRef.current.on(NEW_CHAT_MESSAGE_EVENT, (message) => {
console.log(message);
const incomingMessage = {
...message,
ownedByCurrentUser: message.senderId === socketRef.current.id,
};
setMessages((messages) => [...messages, incomingMessage]);
});
return () => {
socketRef.current.disconnect();
};
}, [roomId]);
const sendMessage = (messageBody) => {
console.log('sendMessage()');
socketRef.current.emit(NEW_CHAT_MESSAGE_EVENT, {
body: messageBody,
senderId: socketRef.current.id,
});
};
return { messages, sendMessage };
};