We are running into the same issue here as well. Been sitting in "Waiting for Review" since early Thursday last week after resubmitting. There is nothing on the App Store Connect status page either that would indicate that this issue exists.

Looks like an issue with App Uploads on App Store Connect was finally acknowledged on https://developer.apple.com/system-status/. We recently migrated to using exclusively Xcode Cloud, so I completely agree that instances like this happen WAY too often with no communication from Apple and are showstoppers for our process. While asking developers to verify that their projects work used to be accurate when dealing with local issues, Xcode Cloud seems to just give out at random times with no indication of what's wrong. These issues seem to solely lie with Apple's implementation of a cloud based build service, which is magnified by a serious lack of communication from Apple during these outages.

We are running into the same issue with our Xcode Cloud instance last night and this morning. It gets all the way through archiving and then gets stuck when the build is getting sent to TestFlight. We started a build last night and it ran for 14 hours with no luck, and we haven't had any builds successfully finish this morning either.