Значение ObservableField внутри ViewModel не обновляет пользовательский интерфейс с привязкой данных

Значение ObservableField<Marker> внутри значения класса ViewModel изменяется с помощью EditText в макете, однако значение не распространяется на TextView tv_summary.

это макет

<?xml version="1.0" encoding="utf-8"?>

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>
        <import type="com.example.tutorial5livedata_mvvm_room_recyclerview.util.BindingUtils"/>
        <variable
            name="viewModel"
            type="com.example.tutorial5livedata_mvvm_room_recyclerview.viewmodel.AddMarkerViewModel"/>

    </data>

    <android.support.constraint.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">


        <TextView
            android:id="@+id/tv_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:text="Title"
            android:textColor="#FC7100"
            android:textSize="18sp"
            android:textStyle="bold"
            app:layout_constraintStart_toStartOf="@+id/guideline_left"
            app:layout_constraintTop_toTopOf="parent" />

        <EditText
            android:id="@+id/et_title"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginEnd="8dp"
            android:layout_marginTop="8dp"
            android:ems="10"
            android:hint="Title"
            android:inputType="textPersonName"
            android:text="@={viewModel.markerObservableField.title}"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="@+id/guideline_left"
            app:layout_constraintTop_toBottomOf="@+id/tv_title" />


        <TextView
            android:id="@+id/tv_latitude"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="8dp"
            android:text="Latitude"
            android:textColor="#FC7100"
            android:textSize="18sp"
            android:textStyle="bold"
            app:layout_constraintStart_toStartOf="@+id/guideline_left"
            app:layout_constraintTop_toBottomOf="@+id/et_title" />

        <EditText
            android:id="@+id/et_latitude"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginEnd="8dp"
            android:layout_marginTop="8dp"
            android:ems="10"
            android:hint="Latitude"
            android:text="@={viewModel.markerObservableField.latitude}"
            android:inputType="number"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="@+id/guideline_left"
            app:layout_constraintTop_toBottomOf="@+id/tv_latitude" />



        <TextView
            android:id="@+id/tv_summary"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="8dp"
            android:layout_marginTop="8dp"
            android:text='@{viewModel.markerObservableField.title + " " + viewModel.markerObservableField.latitude}'
            app:layout_constraintStart_toStartOf="@+id/guideline_left"
            app:layout_constraintTop_toBottomOf="@+id/et_latitude" />


        <android.support.constraint.Guideline
            android:id="@+id/guideline_left"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            app:layout_constraintGuide_begin="8dp" />
    </android.support.constraint.ConstraintLayout>
</layout>

Класс ViewModel

public class AddMarkerViewModel extends AndroidViewModel {

    private MarkerRepository mMarkerRepository;
    public ObservableField<Marker> markerObservableField = new ObservableField<>();


    public AddMarkerViewModel(@NonNull Application application) {
        super(application);

        AppDatabase appDatabase = AppDatabase.getInstance(application.getApplicationContext());

        mMarkerRepository = MarkerRepository
                .getsInstance(MarkerLocalDataSource.getInstance(appDatabase.markerDao(), new AppExecutors()));
        if (markerObservableField.get() == null) {
            Marker marker = new Marker();
            marker.setTitle("New Title");
            markerObservableField.set(marker);
        }
    }


    public long addMarker(Marker marker) {
        return mMarkerRepository.addMarker(marker);
    }
}

onCreate метод Activity для добавления маркера к заданным значениям

   protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        mBinding = DataBindingUtil.setContentView(this, R.layout.activity_add_marker);
        mAddMarkerViewModel = ViewModelProviders.of(this).get(AddMarkerViewModel.class);
        mBinding.setViewModel(mAddMarkerViewModel);

    }