Heirs losing their maternal inheritance.

So I’m playing “A Game of Thrones” mod and have experienced this issue a few times that I don’t remember experiencing in native. Through strategic marriages I’ve set up my heir to inherit other fiefs from their mothers side. I continually check the mothers status and it says that my son is the heir to their titles. This has been as little as a single county and as much as a high lordship. My son is the heir right up until the holder dies and then for some reason switches to another of the mothers family members and my son goes to like 5th or 6th in succession. No warning or event screen explains why. Any idea why this happens or ways to avoid it? Thanks!!