What you're seeing is a scaled down 512x512px icon as the app is downloading. (Developers have always been required to submit a 512px icon for use in iTunes and the App Store on the desktop.) After it's done downloading, it uses the appropriate icon included inside the app bundle. (The 512px icons are not part of the app bundle. They're uploaded separately by developers.)
This is not a bug. This only happens because the developer has not updated the app to include an icon for the Retina display. (I do wonder though, why Apple chose to show the 512px icon during installation. Why not just show the low-res icon during installation too?)