AI rebukes my offer to return a city

[Aramis]

Here's the situation:
Germany declares war on me and attacks one of my cities far from my homeland. I have an MPP with the Byzantines (about to run out), and they declare war on Germany.

While I'm sending units on their way to avenge my loss, the Byzantines carry the burden of the war, losing two cities. I arrive and take those cities from the Germans.

The MPP runs out, but I don't want the Byzs to make a seperate peace and drop out just yet. So I offer a new MPP, which Byz is not interested in. I add in a ton of gold and a tech, and I see "I doubt if they will accept . . ." message. Better than a flat out no, so I offer to give them their 2 cities back. They are insulted by the idea. This seems very odd to me.

The 2 cities had no resources in their areas, and I pulled my units out, so as to not lose them if the Byzs took the offer. What is it about the AI that makes it turn down such a reasonable offer? Lack of resources? Should I have left a unit in each city?

[Solomwi]

It's the fix for an early exploit that involved trading a city to the AI for whatever it had that you wanted, then having that city flip back to you or just retaking it by force. Basically you can now gift a city, but can't trade one. Peace treaty negotiations are the obvious exception to this.

[Aramis]

Hadn't considered that. Never considered the exploit before it was fixed
Of course, this is like the fix creating a new break, which now needs to be fixed.

[Dis]

not a very good fix. but I didn't have high hopes

[MrWhereItsAt]

You will notice a similar thing when extorting techs for peace etc. Sometimes they will almost go for it, so you may like top be magnanimous and offer a little more than just the resigning of the peace accord - at which time they will say they are insulted.

[bongo]

Also noticed that you can only extort each civ *once*.

The second time around they will accept peace but will not give you even 1 gold.

Had a game where an AI attacked me across another AIs territory, as the barrier AI was totally unwilling to let MY troops cross his land I fought a defensive war for centuries. Eventually the AI would give 4-5 tech for accepting peace. Later I took most of the barrier AIs land and started a second war towards the first AI. This time around he wouldn't even give me one tech when I had his last city under siege. I didn't even get close to his territory during the first war.