Games : Why is trade blocked between Gulf of Aden and Alexandria?

I'm playing as Ottomans, and expanded all over the place, giving me pretty nice and long trade routes (which I think is a good thing).

Of course I'm trying to steer as much trade as possible towards Constantinople. I'm pretty dominant in Constantinople, Aleppo, Alexandria, Basra, Persia, Hormuz, and the Gulf of Aden. In all of those I have at least 50% of trade power. (I'm smaller in Ragusa, Tunis and Sevilla'. I´ve got merchants collecting in Ragusa and Sevilla (because a lot of trade value passes through there and I can´t steer it towards Constantinople). I´ve got merchants steering trade in Crimea (because otherwise it´ll go to Kiev), and Alexandria (because there´s also outgoing routes to Genoa and Venice). I´ve got a fifth merchant that still needs a job, but I´m not sure which. Trade from Aleppo can only go to Constantinople and Alexandria, from Basra only to Aleppo, from Persia only to Allepo (man, Aleppo is nice), from Hormuz can co either to Basra or Guldf of Aden, both of which I dominate, and from Gulf of Aden trade can go either to Alexandria (which I want), or to Zanzibar (which I don't want).

And here's the problem: for some reason, no trade is flowing from the Gulf of Aden to Alexandria, despite the fact that the Aden trade node says I'm transferring 5.89 to Alexandria. In the "Outgoing" mouseover, it clearly says 0.00 is going to Alexandria. In Alexandria, the "Incoming" says 0.00 is coming from the Gulf of Aden.

How is this possible? How can my 5.89 turn to 0? Is some unspecified effect causing this? Is it a bug? Do I completely misunderstand how this is supposed to work?