To reproduce
1. Click on menu button. popup will appear very quickly
2. Click again. Nothing will happe
3. Click third time. Now it will open


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

public class BugMenuButton extends Application {
    
    @Override
    public void start(Stage primaryStage) {
       
        MenuButton menuButton = new MenuButton("Menu button");
        
        for(int i = 1; i <= 5; i++) {
          menuButton.getItems().add(new MenuItem("Item - " + i));
        }
        
        HBox root = new HBox(20d);
        root.getChildren().add(menuButton);
        
        Scene scene = new Scene(root, 300, 250);
        primaryStage.setScene(scene);
        
        primaryStage.setTitle(System.getProperty("java.runtime.version") + "; " + System.getProperty("javafx.runtime.version"));
        primaryStage.show();
    }

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