Why SingleSelectionField not "only persists values when explicitly requested" like other fields?

[DQinYuan]

My version:

    <dependency>
      <groupId>com.dlsc.formsfx</groupId>
      <artifactId>formsfx-core</artifactId>
      <version>1.3.1</version>
    </dependency>

For example:

import com.dlsc.formsfx.model.structure.Field;
import com.dlsc.formsfx.model.structure.Form;
import com.dlsc.formsfx.model.structure.Group;
import com.dlsc.formsfx.view.renderer.FormRenderer;
import javafx.application.Application;
import javafx.beans.property.*;
import javafx.collections.FXCollections;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class FormsfxApp extends Application {

    public static void main(String[] args) {
        launch(args);
    }


    @Override
    public void start(Stage primaryStage) throws Exception {

        StringProperty name = new SimpleStringProperty("my name");
        name.addListener((observable, oldValue, newValue) -> System.out.println("new name:" + newValue));
        IntegerProperty age = new SimpleIntegerProperty(10);
        age.addListener((observable, oldValue, newValue) -> System.out.println("new age:" + newValue));

        ObjectProperty<String> selected = new SimpleObjectProperty<>("aaa");
        selected.addListener((observable, oldValue, newValue) -> System.out.println("select:" + newValue));
        ListProperty<String> items = new SimpleListProperty<>(
                FXCollections.observableArrayList("aaa", "bbb", "ccc")
        );

        Form form = Form.of(
                Group.of(
                        Field.ofStringType(name),
                        Field.ofIntegerType(age),
                        Field.ofSingleSelectionType(items, selected)
                )
        ).title("test persist");

        BorderPane borderPane = new BorderPane();
        borderPane.setCenter(new FormRenderer(form));
        primaryStage.setScene(new Scene(borderPane, 1000, 600));
        primaryStage.show();
    }
}

Other Fields like StringField and IntegerField change property only when form.persis() is called explicitly. But selected property in SingleSelectionField is eagerly modified in every time I select other item in combo box.

I find there is a persistentSelection field in SingleSelectionField:

FormsFX/formsfx-core/src/main/java/com/dlsc/formsfx/model/structure/SingleSelectionField.java

Line 50 in cf37703

protected final ObjectProperty<V> persistentSelection = new SimpleObjectProperty<>();

But why it is not exposed for user?