When are foreign trade routes calculated?

[Honkoid]

I can't see a reasonable pattern after which foreign trade routes are calculated. The following happened in a game I just played:

I was Pericles and soon found out, that I was alone on my Island. However I wasn't isolated and met one or two other civs so far (or better, they met me via boats). So I decided to aquire the Great Lighthouse to reap high profits from easy early oversea foreign trade routes. Well, I built TGL and made open borders with the known civs, but there were no foreign trade routes in my cities. Somewhat frustrating. I knew no foreign cities.
The game kept going, I met new civs, but still, no foreign trade routes. One day Bismarck founded a city on the other continent visible to me. I checked my trade routes and found, that I had exactly one foreign trade route now. Should lacking exploration be the source of my problem? Well, let's see, what happened shortly after that.
I switched to Caste System. In the turn of anarchy I meet Gilgamesh. When leaving anarchy, I wonder why my GPT skyrocketed. I checked my cities and voila, I had foreign trade routes en masse from all known civs, though I still only knew that one city Bismarck planted in my visibility range.

Does anyone have an explanation? Mercantilism is still far of for everyone. Looks somewhat buggy to me. And it was also frustrating, because the investment in TGL didn't pay for a looong time.

 

[slaze]

There could have been a barbarian city blocking your access to thier trade network. but without a save, who can be sure

 

[Honkoid]

hmmm, interesting suggestion. I just looked into an autosave some turns before the switch to caste system. I have routes to two AIs but no single foreign trade route. Two saves are attached.

edit: I just opened worldbuilder and you are absolutely correct. Babarians are blocking the coast. But then again I don't understand, why I can trade ressources with two AIs