FragmentActivity onSaveInstanceState not getting called

The issue here is that you are misunderstanding how onSaveInstanceState works. It is designed to save the state of the Activity/Fragment in the case that the OS needs to destroy it for memory reasons or configuration changes. This state is then passed back in onCreate when the Activity/Fragment is returned to / restarted.

In a Fragment, all of their lifecycle callbacks are directly tied to their parent Activity. So onSaveInstanceState gets called on the Fragment when its parent Activity has onSaveInstanceState called.

When pausing the activity (using the back button), the onSaveInstanceState is never called, and consequently, savedInstanceState is always null within the onCreate method upon resuming the app.

When pressing back, the user is destroying the Activity, and therefore its children Fragments, so there is no reason to call onSaveInstanceState, since the instance is being destroyed. When you reopen the Activity, it's a brand new instance, with no saved state, so the Bundle passed in onCreate is null. This is behaving exactly as designed. However, try rotating the device or hitting the home button, then you will see the Activity and its children Fragments have onSaveInstanceState called, and passed back in onCreate when returned to.

The hack you added, directly calling onSaveInstanceState(new Bundle()); inside of onPause, is a very bad practice, as you should never call the lifecycle callbacks directly. Doing so can put your app into illegal states.

If what you really want is for your data to persist beyond an instance of your app, I suggest you look into using SharedPreferences or databases for more advanced data. You can then save your persistent data in onPause() or whenever it changes.

Hope this helps.

In an update to the accepted answer:

A fragment's onSaveInstanceState may be called if you are using a ViewPager with a FragmentStatePagerAdapter (rather than FragmentPagerAdapter)

FragmentStatePagerAdapter

This version of the pager is more useful when there are a large number of pages, working more like a list view. When pages are not visible to the user, their entire fragment may be destroyed, only keeping the saved state of that fragment. This allows the pager to hold on to much less memory associated with each visited page as compared to FragmentPagerAdapter at the cost of potentially more overhead when switching between pages.

And don't forget:

When using FragmentPagerAdapter the host ViewPager must have a valid ID set.