Why does the Firewall Android app sometimes turn itself off?

An internal VPN (hotspot) service is required to run the Firewall Android app.

It is used only to forward and block incoming and outgoing connections (packets) on your Android device.

It is NOT a real VPN service that connects your device to another virtual private network!

As soon as you use or start another VPN service, the firewall is automatically disabled because two VPN services cannot be used at the same time on the Android operating system.