QSpinBox buttons styling broken

As soon as QSpinBox/QDoubleSpinBox is styled with custom borders, its buttons (up and down arrows) gets a 1px blank margin between the borders of QSpinBox and the borders of up/down buttons. No way to remove this margin, even by forcing spacing/margin/padding to 0 to all elements and sub-elements.

This kind of stylesheets create the inner margin problem:

QSpinBox, QDoubleSpinBox {
font-size: 9px;
border: 1px solid rgba(0,0,0,30);
}