I am using MongoDB Realm as a backend for Xamarin.Forms App. There is a scenario where user changes his device or reset it. When he reinstall the app, initially it takes some time to sync the data. This cause confusion.
There should be a mechanism to sync data on device of existing user immediately after login. The sync should happen before the user can use the app.
What you describe is a common use case. It’s actually part of the documentation, titled Display a Progress Bar. What is usually recommended is to show a specific progress bar that blocks the user from interacting with the app and gives the user an understanding of what’s happening.