Why is this node.js app starting a new session on every page view in production, but not in my local environment?

Question

I am making a website using node.js/express. The website works perfectly fine when I run it on my mac, but when I push it to my Digital Ocean Dokku droplet there is a problem with persisting sessions.

Whenever I navigate to a new page locally, the session persists; however, whenever I navigate to a new page in production, a new session is created - and then various other things on the site break as a result.

Initially I was managing sessions using connect-redis, but I have switched it out in favour of express-mysql-session to see if that made a difference (it didn't).

I can also see the new sessions being created in the redis store/database, and so to my mind that rules out the issue being the connection with the database.

I am setting up the sessions in my app.js file as follows:

  // 3. Set up and use sessions

  const sessionStore = new MySQLStore({}, db.pool)

  const sessionMiddleware = session({
    store: sessionStore,
    secret: process.env.SESSION_SECRET,
    resave: false,
    saveUninitialized: false,
    cookie: {
      maxAge: 24 * 60 * 60 * 1000 * 90,
      secure: process.env.NODE_ENV === 'production',
      httpOnly: true,
      sameSite: 'strict'
    }
  })

  app.use(sessionMiddleware)

Is there anything here which might explain what the problem is, or can anyone think of anything in my production environment I should be looking at to try and identify the source of the issue?

Many thanks