Childfragment not updating ActionBar when resumed after back-press in FragmentStatePagerAdapter

Question

I have a small problem with my application which uses a FragmentStatePagerAdapter when scrolling between pages.

First I'll describe how the application is built with a stripped down version.

App description

I have one Activity which holds everything together. This Activity in turn hosts a Fragment, in this case called Pager.

This Pager-class has a ViewPager within, with a FragmentStatePagerAdapter as an inner class which is used to provide the fragments for the ViewPager. The fragments provided to the ViewPager are instances of FragmentA. FragmentA has its own content and its own items on the ActionBar, but can also in turn open other fragments. In my "Real" application this can be done either by clicking some item in a ListView or by clicking a button on the ActionBar. In this simplified case FragmentA can open up FragmentB via a button.

So far this works just as it should.

Problem

When FragmentB is opened from any of the FragmentA-fragments it is properly displayed with its own content and menu in the ActionBar, replacing the menu of FragmentA. However, when navigating back via the back-button the ActionBar for FragmentA is never restored to the state the FragmentA-actionbar had prior to opening FragmentB, instead it's empty.

Possibly strange behaviour

When I change so that MainPagerAdapter extends FragmentPagerAdapter instead of FragmentStatePagerAdapter, all of a sudden it works just fine, and I really don't understand why this is. Also, manually calling supportInvalidateOptionsMenu() did not work at all when the Fragment was contained within a FragmentStatePagerAdapter.

I looked in to the source of both of the adaptertypes but I couldn't figure out why one of them worked while the other didn't.

MainActivity.java

public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        getSupportFragmentManager()
                .beginTransaction()
                .addToBackStack("Start")
                .replace(R.id.container, new Pager())
                .commit();
    }

    public void openB() {
    getSupportFragmentManager()
            .beginTransaction()
            .addToBackStack("Start1")
            .replace(R.id.container, new FragmentB())
            .commit();
    }
}

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
<RelativeLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/container">
    </RelativeLayout>

Pager.java

public class Pager extends Fragment {
ViewPager mViewPager;
MainPagerAdapter mainPagerAdapter;

@Override
public void onCreate(Bundle savedInstanceState) {
    setHasOptionsMenu(true);
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.wrapper_layout, null);
    mViewPager = (ViewPager)v.findViewById(R.id.viewpager);
    mainPagerAdapter = new MainPagerAdapter(getChildFragmentManager());

    mViewPager.setAdapter(mainPagerAdapter);
    return v;
}

private class MainPagerAdapter extends FragmentStatePagerAdapter {

    public MainPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return new FragmentA();
    }

    @Override
    public int getCount() {
        return 2;
    }
}
}

wrapper_layout.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:orientation="vertical"
          android:layout_width="match_parent"
          android:layout_height="match_parent">
<android.support.v4.view.ViewPager
    android:id="@+id/viewpager"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" />

</LinearLayout>

FragmentA.java

public class FragmentA extends Fragment {

@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
    super.onCreateOptionsMenu(menu, inflater);
    menu.clear();
    menu.add("A-Menu");
}

@Override
public void onCreate(Bundle savedInstanceState) {
    setHasOptionsMenu(true);
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.a, null);
    Button b = (Button)v.findViewById(R.id.button2);
    b.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            getActivity().supportInvalidateOptionsMenu();
        }
    });
    Button b2 = (Button)v.findViewById(R.id.button);
    b2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            ((MainActivity)getActivity()).openB();
        }
    });
    return v;
}
}

a.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:orientation="vertical"
          android:layout_width="match_parent"
          android:layout_height="match_parent">
<Button
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="B"
    android:id="@+id/button"
    android:layout_gravity="center_horizontal"/>

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Invalidate"
    android:id="@+id/button2"
    android:layout_gravity="right"
    />
</LinearLayout>

FragmentB.java

public class FragmentB extends Fragment{
@Override
public void onCreate(Bundle savedInstanceState) {
    setHasOptionsMenu(true);
    super.onCreate(savedInstanceState);
}

@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
    super.onCreateOptionsMenu(menu, inflater);
    menu.clear();
    menu.add("B-Menu");
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.b, null);
    return v;
}
}