req.session.user is deleted while user is active

I set the session timeout to 30 minutes. While I am still active, req.session.user is deleted after 30 minutes. However, the session is still alive. Here's my config (i'm using express-session and passport.js):

app.use(session({
    store: new RedisStore(options),
    secret: <some_secret>,
    resave: false,
    saveUninitialized: false,
    cookie: {maxAge: 1800000}
}));

app.use(passport.initialize());
app.use(passport.session());

// Are these serializer/deserializer needed?
passport.serializeUser((user, done) => {
    done(null, user);
});
passport.deserializeUser((user, done) => {
    done(null, user);
});

In login:

router.post('/login', (req, res, next) => {
    passport.authenticate('ldapauth', {session: false}, (err, user, info) => {
        ...
        if (user) {
            req.session.user = {email: req.body.username};
        }
        next();
    })(req, res);
});

The verify code is like this:

isLoggedIn() {
    if (req.session && req.session.user) {
        return true;
    }
    return false;
}

I set the req.session.user to some object after I successfully logged in.

So, after 30 minutes, req.session.user is deleted, but req.session is still there and keeps on incrementing the expiry date since I am still actively working on the page.

Why is req.session.user deleted after 30 minutes? I thought passport rides on the session by express?

from req.session.user is deleted while user is active