Yeah, I caught that it was that the AI started it, and did do my testing by forcing the AI to come to me with the treaty offer. I did have the treaty end the next turn several times while testing, but only if the AI was at war with me when I forced it to give me tribute anyways 
. However, the AI does correctly check that they aren't at war with you before offering the treaty, and changes in relation or player death are about the only things that can cause a treaty to be cancelled before the duration is up. This duration is flat set in the XML, not something the AI can mess with, so if the problem is with your data and you have this happening all the time, any past save should do for me to get a look at your data, without you needing to get the save game into some special state.
The only other info I can think of right now that might help track this down is if you are playing the scenario or not, if you are setting up teams, or any other differences from vanilla randomized sandbox that you play with when this happens. Hopefully something can point in the direction of the bug.
