Cross-platform game using Netcode for GameObjects, Lobby and Relay?

[modern tinsel]

Greetings, I have a multiplayer card game using Netcode for GameObjects, Lobby and Relay. I have a Windows Desktop build and an iOS build (built with Unity Cloud Build). I have a post-build script that pushes the iOS build to App Store Connect where I pull it down to my iPhone/iPad via Test Flight.
Lobby seems to work fine. I can see and join games created by clients on the other platform (i.e. as an iOS user I can see and join games on Windows and vice-versa). However, when the Host starts the game and I call NetworkManager.Singleton.SceneManager.LoadScene("Game", LoadSceneMode.Single); to change scenes, the scene doesn't change for clients on the other platform. It works fine for all clients on the same platform (either iOS or Windows).
I've added some extensive logging and have confirmed that via Relay I am using the same IP and Port on both platforms.
However, it doesn't appear that ServerRPC/ClientRPC traffic is making it across the platforms and as I've pointed out it's not changing scenes across platforms.

What am I missing?

[modern tinsel]

To add this this, I have put together a bare-bones test application using Tarodev's tutorial as a basis and everything works cross-platform so the issue is definitely something specific to my game. How would the experts go about diagnosing this issue? I am able to reproduce locally running the iOS version in my editor and running the a standalone Windows build on the same machine.