iOS 18 upgrade - Unable to Connect to Firestore

[MagicFlow29]

I've just upgraded Xcode, and my actual devices to iOS 18. Some very weird behaviour, and I cannot find anything documented or discussed.

A deployment of my existing code to a simulator or physical device running iOS.17 works perfectly fine... firestore connects.

If I build and deploy to a physical device or the simulator (both with network connectivity) I get the following error:

11.2.0 - [FirebaseFirestore][I-FST000001] Could not reach Cloud Firestore backend. Backend didn't respond within 10 seconds.
This typically indicates that your device does not have a healthy Internet connection at the moment. The client will operate in offline mode until it is able to successfully connect to the backend.

Note -> I've upgraded to the latest 11.20, and this was the same on the earlier 10.29.

Note 2 -> Existing builds of app (from app store), work on iOS 18 devices...

Can anyone help with what I'm missing?