Apparently GIMP is not designed to handle Indexed mode images that contain layer groups. I hesitate to call it a bug because if you add a layer group to an RGB image, the "Image->Mode->Indexed" option becomes inaccessible. And as you noted, dragging every layer out of the folder causes them to become visible. My guess is that the developers are quite aware of this limitation.

I could modify my script to correct for this, but I wish to investigate a little further.

To "fix" your image, first convert it to RGB then add a transparent layer to the layer group (this seems to be the only way to trigger an update of the image). Afterward, you can delete the transparent layer. Note that since your RGB image has a layer group, converting it back to Indexed is no longer an option.

thanks for looking into that !
will do as you say

About what I was expecting. I guess it will have to do for now.

I have substituted a smaller image for the one that was 90Mb so nobody else has to waste 13 minutes downloading it. Thank you for checking it for me.