React App not setting cookie in browser with express-socket.io-session - reactjs

Question

I'm trying to sett a session-id cookie on my client react application. Somehow this does not work and all I get is the standart io session cookie in the browser.
On the other hand it works with a static served HTML file which builds up a socket.io connection.
Here is my server-side code:
var app = require("express")();
var server = require('http').Server(app);
var io = require('socket.io')(server);
let expressSession = require('express-session');
let sharedsession = require("express-socket.io-session");
var path = require('path');
var host = 'process.env.HOST' || '0.0.0.0';
var port = process.env.PORT || 4000;
server.listen(port, host, function () {
console.log("Server running on: " + host + " : " + port);
});
if (process.env.NODE_ENV === 'production') {
//Set static folder
app.use(require('express').static('client/build'));
}
let session = expressSession({
secret: 'my-secret',
//store: new redisStore({ host: 'localhost', port: 6379, client: redis, ttl: 260}),
resave: true,
saveUninitialized: true,
cookie: {
maxAge: 10000,
httpOnly: true,
sameSite: true,
secure: true,
},
})
app.use(function(req, res, next) {
res.header('Content-Type', 'application/json;charset=UTF-8')
res.header('Access-Control-Allow-Credentials', true)
res.header(
'Access-Control-Allow-Headers',
'Origin, X-Requested-With, Content-Type, Accept'
)
next()
})
app.use(session);
io.use(sharedsession(session, {
autoSave: true
}));
io.on('connection', function (socket) {
console.log(`socket with id ${socket.id} connection established`);
socket.handshake.session.userdata = Math.random();
socket.handshake.session.save();
console.log(socket.handshake.sessionID);
client:
let socket = io(serverIP, { autoConnect: false });
useEffect(() => {
socket.on("connect", () => {
console.log("Connected");
});
socket.open();
}, []);
on a simple HTML file like this the session cookie gets set:
<script src="/socket.io/socket.io.js"></script>
<script type="text/javascript">
socket = io();
Both cases are over HTTP. The socket connection also gets established with both variants and the io cookie gets set. And the react App is built and being served from the build folder.
What could I be missing? Thanks for any advice!
EDIT FROM 02.03.2019:
As it turns out the error might come from a missing request or something. When I use a local socket.io.js directly added to the HTML everything works fine. If I use socket.io-client directly imported in my react app or a external CDN it wont work. Seems like it only works after there has been I request to a local ressource. Anyone knows what could be the diference between those two approaches and what could be the reason for this behaviour?