When looking at the settings of an app, the "Follow Global Project Setting" option is missing as shown below:
Release : SAAS
Component : App SDK, Core Apps
This occurs when the page has been timebox filtered in the page settings:
When you set a page to use timebox filtering, it is pulling the timeboxes in the page's dropdown from the currently scoped project. We want all the apps to inherit that particular timebox so they can display the stats for the currently selected timebox. So in a sense, making this setting on the page level forces all apps to use the global project setting automatically and then uses the selected timebox to narrow down the scope of information.
Since projects can have different timeboxes from each other, by having a different project setting option on a particular app, we wouldn't know which timebox to focus on, if any.
This is working as designed
If you wish to see the "Follow Global Project Setting" then you must reconfigure the page to not use timebox filtering. Please see the following video to learn more about working with custom pages: https://www.youtube.com/watch?v=u9u34Ta6opg&t=97s