Why does Protectstar Firewall Android sometimes keep shutting off the protection?

To run the firewall, it needs an internal VPN-(hotspot) service. Unfortunately, it is NOT a real VPN that connects to another VPN.

You cannot use the firewall in addition to another running VPN app (or service). As soon as you start another VPN app on your device, the firewall turns off the protection.

Here is a temporary solution:

You can enable Always-On for the firewall. You can do this in the system settings - Connections - More connections - VPN - Settings wheel for the firewall.