It's definitely a bug that needs to be addressed.
In my latest game I bought tiles near Egypt and he complained about it several turns later. I kindly told him where he could put himself, knowing I'd be buying more tiles later. I did promise not to settle any more cities near him, however.
By mid-game he spammed complaints that I'd broken my promise not to buy tiles near him. I NEVER EVEN MADE THAT PROMISE. I definitely had the negative modifier for it, and he kept complaining about it every 30 turns or so. It was even after I received a notification that my promise to not settle cities near him had concluded (which, again, is not the same promise, but might be related to the bug somehow?).
This needs to be fixed.