canvas.before is only assigned to the last widget kivy

I am writing a DSLR messenger, receiving information from a request. Separate screen with messages when adding a widget:

class text_opponent(Label):
    def __init__(self,tx,**kwargs):
        super(text_opponent,self).__init__(**kwargs)
        self.text = tx
        with self.canvas.before:
            Color(151/255,154/255,170/255,.8)
            self.rect = RoundedRectangle(pos=self.pos,size=self.size,radius=[(20, 20), (20, 20), (20, 20), (20, 20)])

But the canvas is only assigned to the last added widget https://i.stack.imgur.com/9CmvJ.png