Apps disappear from start screen but still in the apps menu

An app can define one or multiple start icons. Each start icon is assigned with a different "start activity" (a different screen in the app).

If an app is reorganized internally this "start activity" may change. As each app icon is assigned to a fixed "start activity" the old app icon you have placed on your home screen (launcher) is no longer valid and is therefore removed.

In the apps menu always all start icons of all apps are displayed. Therefore if the start activity of an app changes the old icon is removed and the new icon is added. Often old and new icon are exactly identical and also the icon text is the same, thus it is not obvious why the icon on the start screen was removed but in apps menu it is still present.