I haven't run into that particular situation. I have only been backstabbed once, and that was by Assyria in my latest game and for perfectly good reasons.
I have, however, noticed that the AI is much more volatile in the late game due to the World Congress and ideologies. When those things really start to come into play, some of my diplomatic relationships fall apart. I think the AI is better at choosing what best suits their interests, and that carries over into diplomacy now. They also react to one another more appropriately, I'm noticing. So, that could be another reason why you're seeing what you're seeing.