Run this sample test :
"
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class MenuButtontest extends Application {

    public static void main(String[] args) {
        Application.launch(MenuButtontest.class, args);
    }

    @Override
    public void start(Stage stage) {

        MenuBar bar = new MenuBar();
        Menu menu = new Menu("test");
        bar.getMenus().add(menu);

        MenuItem item = new MenuItem("lol");
        menu.getItems().add(item);
        
        Button button = new Button("add menu");
        button.setOnAction((ActionEvent event) -> {
            bar.getMenus().clear();
            bar.getMenus().add(menu);
        });
        BorderPane pane = new BorderPane(button);
        pane.setTop(bar);
        Scene scene = new Scene(pane);
        stage.setScene(scene);
        stage.show();

    }
}
"

Now execute the program and run a memory profiler.
Get to live result and inspect the classe "MenuBarButton" inside "MenuBarSkin".

Now every time you click on the button, which remove all Menus and add the same Menu, you have another instance of MenuBarButton created. Nothing is trashed.