A neighbouring ruler had three kingdoms, all under gavelkind succession and limited or no crown authority. Furthermore, he had two duchies, with one county in each. One of the duchies is within the de jure territory to his primary title (his capital), while the second duchy is outside the de jure territory of any of the three kingdoms (but not within the territory of any existing kingdom or empire).
The king had two sons and a couple of daughters. I would expect the two sons to get a duchy (and county) each, and the eldest son taking the primary title and one additional kingdom – thus splitting the realm.
However, when the former king finally kicked the bucket, the eldest son inherited all of his titles, while the younger son remained landless.
Why did this happen?
Actually, the game doesn't list the younger brother in the inheritance anywhere (except for a duchy outside the realm), even though I see no reason for him to be disqualified. He is not a bastard or priest or anything, but he is a brother from another mother (the old King remarried).