Mine was epic speed hence 70 turns I assume you are on normal speed.Is it possible it works differently for direct and third party negociated peace treaties?
Here England will pay 3gpt for 40 more turns, after the treaty expired last turn. View attachment 691607
With that said, it looks like full duration at the initial deal but after 15 turns rip gold.
So yes it works differently but I wonder if it should, something is off, either it should say a 15 turn deal, or it should last 70 turns as it claims.