I spend a whole lifetime upgrading my main tribal holding and when I die I lose all the progress I made and get stuck with the worst province in my kingdom while my other son gets what should be my capital duchy. I don't mind my kingdom being broken up among my sons, but the way it's being split up now is just plain stupid.
From my experience without the update it doesnt go by any 'worst' title. Your heir gets your highest title, and assuming you dont have two of them, the game starts handing out 'equal power' realms out, with the margin of error always going to the second in line or the ones higher in line.
So you have three baddass sites, and a duchy (for simplicity), and good sites without upgrades, and 3 nasty poor sites.
The game gives you the duchy, which is worth some percentage of all the land in it. Once you reach that threshold, then you can inherit other landed titles, but otherwise you only get the duchy and the poorest title (because you
have to hold land). The game will error to every other character before the first heir in terms of giving extra to, and place heirs 2- x before your main heir. Give your other sons land, or take them to battle, or send them off to die in other ways (spymaster) (chaplain).
The game can be manipulated into letting you keep more of the good land by giving out your land carefully, and it helps to try to stick the sons into awkward spots (like having everyone in their court hate them or giving them the wrong types of governments to watch over), but it does also strengthen your top three heirs specifically.
I wish you could plot on their lives . . .