iOS App Offloading
iOS 11 introduced a feature that will “offload” unused apps if device storage becomes low. While user data is maintained, the app will not be visible unless the user restores it to their device via the App Store. If an app is offloaded, it may still receive push notifications, but opening the push will direct the user to the App Store as opposed to opening the full content of message within the app.
This is a global setting and cannot be turned off on a per-app basis. To disable this behavior, perform the following steps:
Open the ‘Settings’ application on your iOS device.
Scroll down and click the ‘App Store’ settings option.
Set the ‘Offload Unused Apps’ switch to off.