Reproducible: always
Is a regression: no
Platform-specific: no
Steps to reproduce (sample attached):
1. Create application with menubar and menu
2. Add hotkey to that menubar
3. Press and don't release ALT+KEY (or other hotkey) to activate the menu

Expected behavior (as native applications): Menu is displayed and remains visible.
Actual behavior: Menu becomes visible for a couple of seconds and hides until hotkey is released.

Sample:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.layout.BorderPane;
import javafx.stage.Stage;

public class MnemonicsFail extends Application{
    
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
         MenuBar bar = new MenuBar();
         Menu root = new Menu("_A menu", null, new Menu("B"), new Menu("C"));
         Menu second = new Menu ("_Other", null, new Menu("Submenu"));
         root.setMnemonicParsing(true);
         second.setMnemonicParsing(true);
         bar.getMenus().addAll(root, second);
         BorderPane pane = new BorderPane();
         pane.setTop(bar);
         primaryStage.setScene(new Scene(pane, 300, 300));
         primaryStage.show();
    }
}