Suicide guarantee plus bonus legacy problem to boot (on what planet would a rational player not take white peace in that scenario).
I saw some of this too.
Where there is a clear problem at the moment (and which hotfix hopefully will reduce) is AI guaranteeing stuff it should conquer, and picking guarantees in cases where it could have gotten an alliance.
Suicide guarantee is basically not that much of an issue, similar to suicide alliances, because you must look at the global power balance "meta game". If the AI cannot beat an enemy the first time around, it's unlikely the situation will be better after it has let the enemy conquer another minor. Sticking to majors solely allying each other is asking for (even more) blobbing. Guarantee is a tool that can be used even if the target doesn't likes you, that's why it's hypothetically useful (ignoring the practical issues that makes it suck).
Guarantees do suck at the moment but that's a game design flaw especially related to it tying up a diplo slot that could be used for a real alliance; I'm hoping we will be able rectify this in not too long time. Getting ahead with AI for it with that assumption in mind.