Why do messenger push notifications on Android stop working when a VPN is enabled?

THE SYSTEM MUTES THE GOOGLE FCM BACKGROUND SERVICE OR THE TUNNEL DROPS THE MESSAGE DELIVERY SOCKET.

Push notifications on smartphones are delivered through a unified system cloud service channel. If the tunneling client intercepts service traffic or breaks a sleeping connection, incoming messages are delayed until you manually open the app.

Breaking Persistent Connections with Notification Servers

To save battery, Android keeps a single background port open to Google Play Services servers. When routing system traffic through an overloaded tunnel server, the session silently times out, causing the OS to stop receiving real-time signals about new incoming events.

How to Restore Timely Notification Delivery

Exclude Google Play Services and Google Services Framework via split tunneling.
Disable aggressive battery optimization and background activity restrictions for your VPN client.
Decrease the Keepalive packet interval in the configuration file.
Allow unrestricted background data transmission in your app settings.