If you have configured your workflows to use 'Latest Release', and there's a Release Candidate available, Xcode Cloud decides to use it.
The problem is that the App Store rejects builds submitted with Release Candidate builds, at least that's what I'm encountering with Xcode 15.3 on the 27th of February, 2024:
This is the error:
Unsupported SDK or Xcode version. Your app was built with an SDK or version of Xcode that isn’t supported. Although you can use beta versions of SDKs and Xcode to build and upload apps to App Store Connect, you need to use the latest Release Candidates (RC) for SDKs and Xcode to submit the app. For details on currently supported SDKs and versions of Xcode, visit: https://developer.apple.com/news/releases.
I'm not really sure if the issue is on Xcode Cloud or on the App Store, but something's up and they don't like each other right now!