Dynastic Stability

So I noticed I still have the issue where-in my grand-child,d who will be my heir after my son, may still change his/her dynasty. It seems to occur in admittedly rare instances in which my grandchild inherits their mother's or grand-father's title (on their mother's side) and for whatever reason that title is currently independent, such as during a mega-war.

For a specific example: marrying Alys Karstark during Clash of Kings and then something happens to Rickard and Harrion before they can make Arnolf the heir to Karhold.