It appears as if this has been fix in beta6

Using the Feedback Assistant - FB19424182

After inspecting the PTP (that I have unfortunately inherited due to a company workforce reduction) I believe I found the issue. It was returning from the startTunnel function before completely setting up the interfaces so we were telling the OS that the VPN was connected when it really wasn't. I suspect we have been getting lucky with all the releases up to now. Once I rewrote the code everything worked.

Another issue I have noticed and I don't know if it's related but on devices that have 18.0.1 or bigger, my VPN app no longer shows up in the Notifications Settings.

I have also gone back and tested the same VPN/PTP client on iOS/iPadOS 18.0 and it works correctly. So there is obviously something I am missing either in the configuration of the PTP that > 18.0 does not like and is therefore shutting down the network.

It's almost as if the system "believes" that network is not working.