This bug was discovered not so long ago by security experts. Apple fixed this bug in iOS 13.4.5, which means that VPN users are no longer in potential danger.
We know that VPN services can't do anything here, because iOS 13.4 doesn't allow them to close existing network connections and force users to reconnect through a tunnel. This error was noticed after the release of the final OS version.
With the release of the new iOS 13.4.5, this problem is solved, but now there is one problem. The new iOS 13.4.5 is a beta version and can only be installed by those who are the developer. The rest will have to wait for the final iOS 13.4.5 version. If you are currently using the iOS 13.4 and want to safely use VPN services, I recommend read this post.