Magic++: Expanded color map not written to file about imagemagick6 HOT 3 OPEN

Image img;
img.read("in.png"); //<-- 8bit paletted file, 255 colors
// read palette
std::vector<Color> colorMap;
for (size_t i = 0; i < img->colorMapSize(); ++i) {
    colorMap.push_back(img->colorMap(i));
}
img.colorMapSize(img.colorMapSize() + 1); //<--after this this the color map is actually 1 entry bigger
colorMap.push_back(Color("#ff00ff"));
for (size_t i = 0; i < colorMap.size(); ++i) {
    img->colorMap(i, colorMap.at(i));
}
img.syncPixels();
// increase color indices in pixels
img.modifyImage();
const auto nrOfIndices = img.columns() * img.rows();
auto pixels = img.getPixels(0, 0, img.columns(), img.rows()); // we need to call this first for getIndexes to work
auto indices = img.getIndexes();
for (size_t i = 0; i < nrOfIndices; ++i) {
    indices[i] = indices[i] + 1;
}
// sync and write modified image
img.syncPixels();
img.write("out.png"); //<--image written has still 255 colors