Yet Another Transparent Proxy Provider Issue: The IPSecining

This one is sorta behaving similar to the FaceTime / AirDrop issue, but it does depend on order, which makes me wonder if it's a programming choice. Specifically, using FortiNet's VPN client, using IPSec, if I have a TPP installed and then try to connect it, it fails. If, however, I connect and then start the TPP, it succeeds, which at least makes it better than FaceTime and AirDrop.

So my question here is... hm, not as well-articulated as I would like. I'm curious if a VPN can check to see if other VPNs are installed and configured, and if so say "nope." Hm, saying that more clearly: I think it's possible for a network extension to check the interface that a packet/flow is going to, and cause a failure of some sort if it's a VPN, correct? Does anyone do that? Or am I seeing lions in the waterhole weeds?

I'm also curious if Apple's networking code has issues with multiple VPNs. (Although, I will note, our TPP works just fine with Tailscale, so it's not an inherent conflict. Also Cisco AnyConnect. So maybe it's just IPSec?)

ETA: to make it clear, my test case involves using a ****** TPP, where handleNewUDPFlow and handleNewFlow both immediately return false, meaning that the system should behave as if it's not there, and yet... doesn't.

I appreciate any comments/assistance/guffaws.