为什么Kivy在画布上只为创建的最后一个按钮绘制三角形？

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.floatlayout import FloatLayout

class MyLayout(FloatLayout):
    def __init__(self, **kwargs):
        super(MyLayout, self).__init__(**kwargs)
        
        # 创建按钮并设置绘制顺序
        button1 = Button(text='Button 1', size_hint=(None, None), size=(100, 50))
        button1.pos = (100, 100)
        button1.canvas.after.add(PushMatrix())
        button1.canvas.after.add(Rotate(angle=30))
        button1.canvas.after.add(Triangle(points=[button1.x, button1.y, button1.x + button1.width, button1.y, button1.x + button1.width/2, button1.y + button1.height]))
        button1.canvas.after.add(PopMatrix())
        self.add_widget(button1)
        
        button2 = Button(text='Button 2', size_hint=(None, None), size=(100, 50))
        button2.pos = (200, 200)
        button2.canvas.after.add(PushMatrix())
        button2.canvas.after.add(Rotate(angle=30))
        button2.canvas.after.add(Triangle(points=[button2.x, button2.y, button2.x + button2.width, button2.y, button2.x + button2.width/2, button2.y + button2.height]))
        button2.canvas.after.add(PopMatrix())
        self.add_widget(button2)

class MyApp(App):
    def build(self):
        return MyLayout()

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