Stylesheet :hover not working after frameless window drag&drop

[nkiy]

Hi guys, I have a frameless window (with flags: Qt::FramelessWindowHint and Qt::WindowCloseButtonHint), and there are two buttons which are used as minimize button and close button.
I set those two QPushButton stylesheet:

QPushButton {
    border: none; outline: none;
    image: url(:/images/close_normal.png);
}
QPushButton:hover {
    image: url(:/images/close_hover.png);
}
QPushButton:pressed {
    image: url(:/images/close_pressed.png);
}

When the window first pops up, everything worked fine, but after I move the window by dragging, the ':hover' style not working any more, and then I click the window, ':hover' worked fine again.
So it's like the window's losing focus when I drag it, I don't know why this happening, any one could help?

Platform: Archlinux KDE, Qt Version: 5.11.0