Magic++: Expanding the color map destroys it

[HorstBaerbel]

Using Magic++ 6.9.7.4+dfsg-16ubuntu6.4 amd64 on Ubuntu 18.04.2.
When I load a 8-bit paletted PNG, resize the color map and write the file again the palette is destroyed. This is due to AcquireImageColormap() being called internally. The color map should at least retain the minimum possible number or colors.
Example code:

Image img;
img.read("in.png"); //<-- 8bit paletted file
img.colorMapSize(img.colorMapSize() + 1);
img.write("out.png");

[HorstBaerbel]

Is this a bug, or am I "doing it wrong"? I am simply trying to add a color to the color map.