LoneRebel
Emperor
Yeah, this annoys me that Natural border growth apparently counts. Definitely a glitch.
But I solved it by not only breaking my "promises", but breaking my foot up their... context sensitive.
Yes, that is always the best way to solve problems with the AI.
I gotta say though, like TheGrumpyBuddha, I don't think this has happened to me yet. I always promise not to buy more tiles and not to settle near them again, and I always keep those promises. (I may buy more tiles and settle near them again after the duration of the border promise expires.) I don't think this has happened to me yet, this natural culture expansion triggering a breaking of either promise.