This issue eventually just went away. I received a fairly opaque reply from Apple through the Feedback Assistant about 9 days after the issue first appeared that read: There were some system issues last week that have since been addressed. Please attempt to reproduce again and let us know if this still occurs. I don't believe there's any more to be said on this issue.

Thanks endecotp. I've read through the information on the thread post https://developer.apple.com/forums/thread/767002, but the presenting error there is related to provisioning profiles, rather than this 'validateAssets' issue (though there's indications that it's actually the same underlying issue), and also more specific to Fastlane. The workarounds in that other thread don't apply to much situation.

I find the interface in Xcode to edit these .plist files really clunky (and the 'reasons' for using an API aren't up to date and also don't filter according to the API you've selected). I own an app development company and have to create like 50 of these in the next few weeks. So I built a form to generate them really quickly and have published it so anyone can use it: https://wemakeapps.net/manifest-maker Enjoy. (PS Sorry it's not a real link, Apple won't let me)

In the end, this issue just went away on its own after about 3 days. We changed nothing at our end. Seems like some little issue at the Apple web-server end.

Thanks App Store team for the hint. I've submitted a report through that channel. Is there anyone else out there that has any hints on how to possibly solve this problem?