Yeah this is a bug all right.

To cap a city once you are sending loyalty waves at the quickest takes roughly 1hr 45mins. As we all know after loyalty is below 15 you time your attacks 6mins apart.

Simple, but recently I've known people end up hitting a city once it's loyalty is at 1 for nearly an hour before they finally cap it.

If that ain't a bug I don't know what else is

I've heard of people seemingly remaining in truce for days before, think this a glitch that some have unscrupulously exploited.