In iOS 18, I've noticed that the clipToBounds setting for UINavigationTransitionView is turned "OFF." What could be the reason? In previous versions (checked up to iOS 15), it was enabled, and direct access to UINavigationTransitionView isn't provided.
What are you trying to resolve? The settings of various private views are usually chosen to realize the desired UI design that the framework is trying to provide and is why their specific settings are not defined as API behavior.