Looking at the code, it looks like they are not logging into MongoDB/Realm async, they are just checking if it is done, then reloading the page if not.

    const user = app.logIn(credentials);

    export const client = num_users===0 ?  
    // wait for 1 second before reloading the page
    setTimeout(() => {
      window.location.reload();
    }, 350) : app.currentUser.mongoClient("mongodb-atlas")

(which coincides with loads of realm users in localstorage if it continues to reload)

This https://www.mongodb.com/docs/realm/web/authenticate/ shows how to login whilst waiting for a user to be created.

Edit: @aditiyadav Hope that doesn't come across wrong, just pointing out where the problem could be :)