As the King of Aquitaine, I had a vassal Countess of Foix. Her heir was Count of a single county in France (both nations are Elective Gavelkind). I had a Vassal Inheritance Warning, which makes sense; when the countess dies, her title would pass to her heir, who would add it to his French title, that being older.
So I created the Duchy of Toulouse and granted it to my Countess of Foix. I figured that this way, when she died, both titles would pass to her heir, and he'd bring his French county to over to join his higher Aquitainian Dukedom.
But when Comtesse du Foix / Duchesse du Toulouse kicked the bucket, I lost Foix to France, and another vassal held Toulouse. Is this an effect of the succession laws? Did one title pass to one heir, and the other to another heir? Could I have seen that, and prevented it somehow?