Run the code:

    @Override
    public void start(Stage stage) throws Exception {
        Accordion acc = new Accordion();

        TitledPane pane1 = new TitledPane();
        pane1.setText("Some _text");
        pane1.setMnemonicParsing(true);
        pane1.setGraphic(new Label("Node"));
        pane1.setContent(new Button("Some _button"));
        acc.getPanes().addAll(pane1);

        VBox pane = new VBox();
        pane.getChildren().addAll(acc);
        Scene scene = new Scene(pane, 300, 700);
        stage.setScene(scene);
        stage.show();
    }

When I expand node and press alt, i see underlined "b" in button, but "t" is not underlined. But, alt+b alt+t transfers focus from button to titledPane and back.