In my current Russian campaign I experienced a similar problem.

I was Outstanding with the Danes and had an alliance since nearly the beginning of the game. They went to war with Poland and, out of the goodness of my heart ( ) I "gifted" them help in their war by saying I'd give them support. That coupled with the first assault gave me Perfect relations.

I just so happened to have a princess nearby a Danish general. My high charm princess nabbed him up quick. So, I trotted him along, but then ran into some Poles blocking my way with two small Danish stacks on the other side. What do they do? They attack me!!! This plunged us into war and I haven't been able to get a ceasefire since.

So far the diplomatic AI is just as poor as it used to be for me, although I did notice that when I steal generals with princesses I don't hurt relations when I travel him through their lands. Other than that, it's still poor.