To add more confusion, it seems that iPhone 14 Pro devices running TestFlight builds made with Xcode 14 are behaving as if they were Xcode 13 builds. That is, the iPhone 12/13 Pro layout is being scaled to fill the larger screen of the 14 Pro devices. This explains why Simon Støvring was seeing different behaviour when running through Xcode or a TestFlight build. Hopefully that will be fixed soon, but Anton Sotkov has seen it in Xcode 14.0 and 14.1 beta 2, and with iOS 16.0 and iOS 16.1 beta 2.
This results in Unread having a strange layout when installed via TestFlight. The same build works as expected when installed via the App Store or when run on an iPhone 14 Pro simulator from Xcode 14.
More generally, it is a serious problem that the behavior of a build when installed via TestFlight is so different from that same build when installed via the App Store.