QImage::setPixel() does not work on Format_Grayscale8

QHash<QRgb, qint32 (*)(QRgb)> colorFuncs {
	{ Qt::red,   qRed},
	{ Qt::green, qGreen},
	{ Qt::blue,  qBlue},
	{ Qt::gray,  qGray},
};

QImage color8(const QImage &image, int channel, QImage::Format outFormat) {
	QImage out(image.width(), image.height(), outFormat);
	int (*colorFunc)(QRgb rgb) = colorFuncs[channel];

	if (QImage::Format_Indexed8 == out.format()) {
		QVector<QRgb> values;
		values.reserve(256);
		if (channel==Qt::gray) { for (int c=0; c<256; c++) values.append(qRgb(c,c,c)); }
		if (channel==Qt::red)  { for (int c=0; c<256; c++) values.append(qRgb(c,0,0)); }
		if (channel==Qt::green){ for (int c=0; c<256; c++) values.append(qRgb(0,c,0)); }
		if (channel==Qt::blue) { for (int c=0; c<256; c++) values.append(qRgb(0,0,c)); }
		out.setColorTable(values);
	}
	for (int y=0; y < image.height(); y++) {
		for (int x=0; x < image.width(); x++) {
			out.setPixel(x,y, colorFunc(image.pixel(x,y)));
		}
	}

	return out;
}

QImage gray8 = color8(image, Qt::gray, QImage::Format_Grayscale8);
QImage index = color8(image, Qt::gray, QImage::Format_Indexed8);

int differences = 0;
int same = 0;
for (int y=0; y < gray8.height(); y++) {
	for (int x=0; x < gray8.width(); x++) {
		//qDebug() << "gray" << gray8.pixel(x,y) << "index" << index.pixel(x,y);
		if (gray8.pixel(x,y) != index.pixel(x,y) )
			differences++;
		else
			same++;
	}
}
qDebug() << "differences" <<  differences;
qDebug() << "same" <<  same;