If the background is transparent, PrimaryScreen().grabWindow(self.winId()) Question.

I'm trying to use the primaryScreen().grabWindow(self.winId() to implement a screenshot, but when the background is not clear, the screenshot is good.
If the background is transparent, the stored image is saved as a black image. Why is this happening?

Code:

import os , sys , time
from PyQt5 import QtWidgets
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *
import numpy as np
 
class MyApp(QWidget):
 
    def __init__(self):
        super().__init__()                
        self.setWindowFlags(Qt.WindowStaysOnTopHint | Qt.FramelessWindowHint)        
        self.setAttribute(Qt.WA_TranslucentBackground)   
        self.initUI()
 
    def initUI(self):  
        self.setGeometry(30, 30, 300, 300)        
        self.setWindowTitle('blRect')        
        self.show()
 
    def screenshot(self):             
        self.preview_screen = QApplication.primaryScreen().grabWindow(self.winId() )               
        self.preview_screen.save('test.jpg', "jpg")
 
    def paintEvent(self, e):
        qp = QPainter()
        qp.begin(self)       
        self.draw_rect(qp)        
        self.screenshot()
        qp.end()
 
    def draw_rect(self, qp):
        qp.setBrush(QColor(255, 255, 0))
        qp.setPen(QPen(QColor(0, 0, 0), -1))
        qp.setBrush(QColor(0, 0, 0))    
        # print(self.width() , self.height())    
        for i in range(3):
            rand_x = 100 * np.random.randn()
            rand_y = 100 * np.random.randn()
            qp.drawRect(self.width() /2 + rand_x, self.height() / 2 + rand_y, 100, 20)
 
def main():
    app = QApplication(sys.argv)    
    ex = MyApp()
    sys.exit(app.exec_())
 
if __name__ == '__main__':
    main()