Fragment not receiving LiveData updates after remove + add

I am currently playing around to get a hang of the fragment's lifecycle in relation to ViewModel and LiveData.

I have 2 fragments, fragmentA and fragmentB. I add the Observer in the onCreate method of each fragment.

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    sharedViewModel = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
    sharedViewModel.getText().observe(this, new Observer<CharSequence>() {
        @Override
        public void onChanged(CharSequence charSequence) {
            editText.setText(charSequence);
        }
    });
}

Each fragment has a button that changes LiveData in the shared ViewModel

public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    [...]

    buttonOk.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            sharedViewModel.setText(editText.getText());
        }
    });

    [...]
}

SharedViewModel:

public class SharedViewModel extends ViewModel {
    private MutableLiveData<CharSequence> text = new MutableLiveData<>();

    public void setText(CharSequence input) {
        text.setValue(input);
    }

    public LiveData<CharSequence> getText() {
    return text;
    }
}

When I click a button, I replace the fragment for the other one.

public class MainActivity extends AppCompatActivity {
    Fragment fragmentA = new FragmentA();
    Fragment fragmentB = new FragmentB();

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container_a, fragmentA)
                    .commit();
        }
    }

    public void switchToA(View v) {
        getSupportFragmentManager().beginTransaction()
                .replace(R.id.container, fragmentA)
                .commit();
    }

    public void switchToB(View v) {
        getSupportFragmentManager().beginTransaction()
                .replace(R.id.container, fragmentB)
                .commit();
    }
}

Replace causes the fragment to be fully destroyed and run through it's onCreate method again the next time it is added. I can confirm that onCreate is called for each fragment placed onto the screen and the Observer is added. But once I replaced a fragment and re-added it, it completely stops getting any updates in onChanged. Even the ones it sent itself. onChanged is just not triggered anymore. I don't understand why.

Edit:

I actually found out that the followig if check in the LiveData class returns the 2nd time I try to add the Observer (after replacing the fragment for the first one):

@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
    assertMainThread("observe");
    if (owner.getLifecycle().getCurrentState() == DESTROYED) {
        // ignore
        return;
    }

Hence, the Observer is not added anymore. Why does getCurrentState() return DESTROYED when I try to re-add the fragment?

In short: the Observer is removed when the fragment is removed, but it doesn't add another Observer the next time the fragment is added.