Newlines in promptText are ignored.

Since the texarea ist a multiline input the prompt text should also support that.

Workaround: use \r instead of \n

Example:
{code}
public class Test extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {
        TextArea textArea = new TextArea();
        textArea.setPromptText("11111111\n2222222\n33333333");
        primaryStage.setScene(new Scene(textArea, 300, 250));
        primaryStage.show();;
    }

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