Transparent Proxy overrides flow.metaData.sourceApplicationSigningIdentifier property

Hello,

I'm developing a transparent proxy which only intercepts traffic coming from certain apps.

I'm having a problem when there are other transparent proxies active where the flow.metaData.sourceApplicationSigningIdentifier property is whichever provider intercepted the traffic before my provider did.

To verify this, I have implemented a small application that installs two transparent proxy profiles which handle the flows only coming from Safari.

Here's the is the bit of the code where the provider determines that:

    open override func handleNewFlow(_ flow: NEAppProxyFlow) -> Bool {
        guard let flow = flow as? NEAppProxyTCPFlow else { return false }

        let sourceApp = flow.metaData.sourceAppSigningIdentifier
        NSLog("[TransparentProxyProvider] Received flow from: \(sourceApp)")

        guard sourceApp == "com.apple.Safari" else { return false }

        // Create NWConnection and handle flow copying as needed

        return true
    }

As you can see from the following screenshots, when both profiles are active are the same time, the logs show that the second profile sees that the source application is the first profile:

From what I understand, that happens because the Transparent Proxy Provider creates a TCP connection and therefore, from the Operating System's perspective, is initiating a new separate flow which is what is then intercepted by the second provider.

My questions are:

1. Is this expected behavior?
2. Is there a way to find what the actual source application was?
3. How does the Operating System determine which profile receives the traffic first?