To reproduce push Alt key.
The line under the 'B' button is expected to remain but it disappears.

public class MnemonicsTest extends Application {
    
    @Override
    public void start(Stage primaryStage) {
        
        Parent root = createScene();
        
        Scene scene = new Scene(root, 300, 250);
        
        primaryStage.setTitle(VersionInfo.getRuntimeVersion());
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

    private Parent createScene() {
        
        Button button = new Button("_Button");
        button.setMnemonicParsing(true);
        
        HBox root = new HBox(5);
        root.getChildren().add(button);
        
        return root;
    }
}