Unable to turn off VPN without disconnecting WiFi after iOS 18.2 update

If you are using a third-party VPN App, there is strong possibility that you have configured Connect On Demand somewhere in the individual App settings. This configuration setting may be worded as something else by the App (e.g., Kill Switch, Persistent Connection etc.) but it will have the same effect. 

When so configured, the VPN App will try to ensure that your network is always protected by the VPN - as such, it will restart the VPN if has disconnected for any reason. When this occurs, you’ll see the VPN icon appear - and possibly the VPN App.

From where are you attempting to control your VPN connection? Would this be from within a VPN App, iPad settings, or from the Control Centre?

You're very welcome. I'm delighted to learn that the source of the problem has been identified and resolved.