GdkPixbuf: ICO files with multiple sizes

This issues is related to this mailinglist posting https://mail.gnome.org/archives/python-hackers-list/2019-June/msg00000.html where I was recommanded to open a bug report for it.

Short: Loading a ico-file containing different resolutions as a GdkPixbuf.Pixbuf causing picutre noise like on an old TV.

This is about PyGobject with Python 3.

Please see this example ico file. I am not sure if the problem is me or the ico file itself is not valid.

In Firefox the icon looks like expected. When I open the (with aiohttp) downloaded ico file with IrfanView (via wine on Debian stable) or with Gimp it looks like expected. Especially in Gimp I can see the icon in multiple sizes (each in one GIMP layer). Fine.

I want to open it with GdkPixbuf "scale" it to 16x16 (or select the correct size). I want to use this Pixbuf as an icon in a Gtk.TreeView and I want to store it back into an ico file.

This doesn't work because the scaled result looks like picture noise on an old TV.

What I tried so far...

org = GdkPixbuf.Pixbuf.new_from_file('_favicon.ico')
small = org.scale_simple(16, 16, 2)
small.savev('small.ico', 'ico', '', '')

and

org = GdkPixbuf.Pixbuf.new_from_file_at_size('_favicon.ico', 16, 16)