Unexpected black border around the object painted by paintEvent() of a frameless QWidget with translucent background in dark mode

Reproducer attached. The dark mode can either set by

void callNSMethod() {
  [NSApp setAppearance:[NSAppearance
                               appearanceNamed:NSAppearanceNameDarkAqua]];
}

as what is done in reproducer, or by system "Appearence" setting. Either can trigger the issue.

The issue is that the rectangle painted inside paintEvent() is somehow surrounded by black border, even though there is no code for such border. There is no such border in light mode.

It is said that unsetting Qt::FramelessWindowHint also solves the border problem in dark mode.

2 screenshots are also attached to illustrate the problem.