The image acuired by QOpenGLWidget::grabFrameBuffer is different from the image displayed on the screen

Reproducer attached.

The image acuired by QOpenGLWidget::grabFrameBuffer is different from the image displayed on the screen.
Attached png files are showing the difference:
 -  screenshot.png : A screen shot of the application window.（The triangle is filled with gray)
 -  paste_from_clipboard.png : An image acquired with grabFramebuffer and stored in the clip board.（The triangle is partly filled with white, and the rest is transparent）
 -  sample.png : An image acquired with grabFramebuffer and directly stored as a file.（The triangle is transparent)