Surely that's not the only icon you're supplying for iOS, right? iOS requires many icons in order to work correctly. Please share the entire list of icons you're using for iOS, or if the above is all you're supplying, let us know that for sure.
In the meantime, see: Customize app icons - Apache Cordova
You're correct, I left the other declarations out for brevity. My point is that the exact same declaration works on CLI 5 and not on CLI 6, so I can only assume it is correct unless the usage has changed from 5 to 6?
Not necessarily -- 5 to 6 is a breaking change, so it depends on how you've got your config.xml configured and how you have your project structured. Can we see the folder structure of the zip file you're uploading to PGB?
I would suggest switching from <icon ... gap:platform="ios" ... /> to something like this instead:
<icon ... />