There appears to be different behaviour for apps that use the UIScene life-cycle compared to those that do not. For apps that do not support scenes, UIApplicationMain() is called and the AppDelegate is initialized, but application:didFinishLaunchingWithOptions: is not called during prewarming. For apps that do support scenes, application:didFinishLaunchingWithOptions: is called during prewarming, although no scene is created - -[UISceneDelegate scene:willConnectToSession:options:] and other methods are not called. Hopefully  can confirm whether this is intentional behaviour and make the documentation more accurate.

Following the instructions from @drewcrawford works - crash diagnostics are delivered to apps immedately after they relaunch and register a subscriber. Hang diagnostics, though, never seem to be delivered - at least not for simple cases of blocking the main thread for 5 seconds. Does anyone have insights into why they are not being received?