Why doesn't my heir inherit my wife's titles?

So I'm playing as the Emperor of Britannia, and I'm trying to get Scotland into the fold at the moment. So I thought hey, let me marry my son to the duchess of Moray so that his son, my future character can inherit her titles as well as mine (I have done this before).

But now I'm playing as my son, who is still married to the duchess of Moray (so now my wife), and they have had three kids, two girls and a boy last. Now the boy is my heir, obviously as my succession laws are Agnatic-Cognatic Primogeniture, but for some reason my wife's heir is my daughter, with the second in line being my other daughter, and the third in line is some other random I've never heard of!

I've checked her succession laws and she is also Agnatic-Cognatic Primogeniture, so her male child should be her heir? I don't understand.