Icon not showing when installing B2B app

I've built an app for B2B distribution. The client then uses the VPP portal to generate install codes for the app. However, when someone redeems that code in the App Store app on their iPad, the dialog that comes up to confirm that the code has been redeeemed has a grey box instead of the app icon. Image here: https://www.dropbox.com/s/l86lzb9ce03xd51/image2.PNG?dl=0


I've set up the icon in iTunes Connect. It's provided in the asset catalog as a 1024x1024 icon. Also it's included separately as a 512x512 png in a file called iTunesArtwork and as a 1024x1024 png called iTunesArtwork@2x, as that's suggested in some documentation for ad-hoc apps. I don't know how else I could possibly provide it, so I'm hoping someone may have another suggestion, or some other way in which this may be failing to show up.