In my current playthrough, I've had two situations relative to closed borders:

- with the Cravers, while in Cold War and with closed borders enacted, my trade routes (from my HQ to my Subsidiaries) are inexistant; nothing appear on the trade map, and no routes are listed in the trade screen

-with the Empire, I was at war unti I forced truce; I'm now in the stage where no other negotiation can be done, the borders are thus closed, but my trade routes are "working": 

they are shown as blockaded in the trade map overview but they are listed as properly working in the trade screen, where no blockade at all are signaled

I can't find any documentation related to that, so I'm unsure if it's a bug or a designed mechanic, or if it's a special rule applied to Truce situations. 

Any help most welcome !