I've seen this happen frequently before, although usually outside of the realm I was in. I arrange a betrothal - specifically, a matrilineal one between one of my daughters and someone, say a French duke's younger son (and I'm playing, say, someone in Ireland). They accept the betrothal. Then the dude's older brother passes away, leaving this younger one to inherit the whole thing, so when they come of age and are ready to marry, when I contact the duke, he'll refuse, saying that so-and-so is too high in the line of inheritance. But he'll contact me, eventually, asking for that same matrilineal marriage.
I'm guessing that there's one thing controlling how the AI handles offers, and another one that handles how they keep their word?