Cannot get value of req.user for Passport.js

Question

I spent hours figuring things out why I cannot get the value of req.user when Passport.js serialized a user. But magically, when I deleted the database collection that holds the session, it worked again.

My stack:

• Vue.js
• Express
• Mongoose MongoDb (I store my data on Atlas)
• Node.js

I use express-session and connect-mongo to create and save session data and use it to serialize and deserialize user using Passport.js

App.js:

const session = require("express-session");
const passport = require("passport");
const MongoStore = require("connect-mongo")(session);

// Sessions
app.use(
  session({
    secret: "this is a sample secret",
    resave: false,
    saveUninitialized: false,
    store: new MongoStore({ mongooseConnection: mongoose.connection }),
  })
);

//Passport Middleware
app.use(passport.initialize());
app.use(passport.session());

Then I call req.user on a route like this:

router.get("/users", async (req, res) => {
  try {
    if (req.user) {
      res.send(req.user)
    } else {
      res.send("no-user-found",)
    }
  } catch (err) {
    console.error(err);
  }
});

I'm calling /api/users on the front-end with Vue.js and Axios hosted on localhost port 8080. Also, tested on the server itself by calling http://localhost:3000/api/users

Now it works, now that I have deleted the sessions database collection on MongoDb Atlas.

I'm just wondering why this happens? Will it repeat again in the future?