QPixmap::transformed() returns bad pixmap

#include <QApplication>
#include <QColor>
#include <QIcon>
#include <QHBoxLayout>
#include <QPixmap>
#include <QToolButton>
#include <QWidget>

int main(int argc, char **argv)
{
	QApplication app(argc, argv);
	QWidget *window = new QWidget;
	QHBoxLayout *layout = new QHBoxLayout(window);
	QToolButton *btn = 0;

	btn = new QToolButton(window);
	QPixmap p1(20, 20);
	p1.fill(Qt::black);
	p1 = p1.scaled(15, 15);                                 // here <--v
	btn->setIcon(QIcon(p1));
	btn->setToolTip("black");
	layout->addWidget(btn);

	btn = new QToolButton(window);
	QPixmap p2 = QPixmap(20, 20);
	p2.fill(Qt::red);
	p2 = p2.scaled(15, 15); // if the size is exactly the same as here ^,
	                        // a bug is triggered (we get black pixmap here)
	btn->setIcon(QIcon(p2));
	btn->setToolTip("red");
	layout->addWidget(btn);

	// similar example:
	btn = new QToolButton(window);
	QPixmap p(20, 20);
	p.fill(Qt::green);
	p = p.scaled(25, 25); // if it was QSize(15, 15), we would get black pixmap
	btn->setIcon(QIcon(p));
	btn->setToolTip("green");
	layout->addWidget(btn);

	btn = new QToolButton(window);
	p = QPixmap(20, 20); // without this line the bug isn't trigerred (I don't get it...)
	p.fill(Qt::yellow);
	p = p.scaled(25, 25); // we get green pixmap here
	btn->setIcon(QIcon(p));
	btn->setToolTip("yellow");
	layout->addWidget(btn);

	window->show();
	return app.exec();
}