First menu item is selected if the drop down menu is expanded first time. To reproduce:
- expand drop down menu using mouse
First menu item will be selected unexpectedly. Note that there is no selection at the next attempt.

Sample to check:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class App extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        SplitMenuButton control = new SplitMenuButton();
        control.setText("Split Button");
        for (int i = 0; i < 5; i++) {
            control.getItems().add(new MenuItem("Item " + String.valueOf(i + 1)));
        }
        Scene scene = new Scene(new HBox(control));
        stage.setScene(scene);
        stage.show();
    }
}