QScrollArea isn't auto-scrolling on QRadioButtons

[AlaaM]

I have some widgets inside a QScrollArea:

scrollArea     = new QScrollArea(&widget);
groupBoxLayout = new QVBoxLayout();
groupBox       = new QGroupBox();
title          = new QLabel();

groupBoxLayout->addWidget(title);

for(int i=0; i<WIDGETS_NUMBER; i++)
{
    buttons[i] = new QPushButton();
    buttons[i]->setFocusPolicy(Qt::FocusPolicy::StrongFocus);
    buttons[i]->installEventFilter(this);
    groupBoxLayout->addWidget(buttons[i]);
}
buttons[0]->setFocus();

Now as long as the buttons[] are QPushButtons, the QScrollArea scrolls automatically on focus change. That is, when I move through the buttons using the Down arrow key, just when I reach the bottom edge of the window, it scrolls down automatically. But just when I change those buttons[] to be QRadioButtons instead, it stops scrolling automatically. Why is that happening? How can I get it to auto-scroll when the focus reaches the edge?

Of course I can detect a FocusIn event, and manually scroll whenever I need:

scrollArea->verticalScrollBar()->setValue(scrollArea->verticalScrollBar()->value()+SCROLL_VALUE);

but I want it to be automatic.