I stored my login credentials in the app. App is asking me to enter them again.
Once you enter your BigCommerce storefront credentials and login successfully, the app stores the credentials in your device local storage for easy repeat login.
In a rare scenario, if you device storage is running low in space, the operating system (iOS and Android) clears off app local storage to make space. Though the app uses very little local storage, the app credentials gets cleared off in this operating system driven operation. Once you enter your credentials again, the app will validate the credentials and store it again for future use.