为什么Kivy Button不能传播事件？

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.label import Label
from kivy.uix.boxlayout import BoxLayout

class MyButton(Button):
    def on_touch_down(self, touch):
        # 调用父类的on_touch_down方法处理触摸事件
        if super(MyButton, self).on_touch_down(touch):
            return True

        # 自定义事件处理逻辑
        print("Button被点击了！")
        return True

class MyApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        button = MyButton(text='点击我')
        label = Label(text='请点击按钮')
        layout.add_widget(button)
        layout.add_widget(label)
        return layout

if __name__ == '__main__':
    MyApp().run()