Why is iconutil generating chopped up images on retina display?

I have an .iconset which can be found here. Building a binary with Xcode results in an icon that looks like

Generating a .icns file directly using iconutil results in the same thing. Everything but the full resolution asset look weirdly chopped up on retina screens, but displays fine on non-retina screens. Converting the generated .icns back to an .iconset gives me the chopped up assets instead of the original assets.

Anyone have any idea as to what might be going wrong here?

I figured it out, though I'm not exactly sure what caused it.

I generated the app icon using Gimp. I believe when I first generated the icon I created a 1024x1024 icon and then scaled it down using Preview. I just regenerated all of the assets by scaling and exporting all through Gimp. Now the assets look fine. Presumably some information was lost in Preview's scaling.