Territory lost through Vassal Inheritance - what should I have done? [duplicate]

I'm confused.

As the King of Aquitaine, I had a vassal Countess of Foix. Her heir was Count of a single county in France (both nations are Elective Gavelkind). I had a Vassal Inheritance Warning, which makes sense; when the countess dies, her title would pass to her heir, who would add it to his French title, that being older.

So I created the Duchy of Toulouse and granted it to my Countess of Foix. I figured that this way, when she died, both titles would pass to her heir, and he'd bring his French county to over to join his higher Aquitainian Dukedom.

But when Comtesse du Foix / Duchesse du Toulouse kicked the bucket, I lost Foix to France, and another vassal held Toulouse. Is this an effect of the succession laws? Did one title pass to one heir, and the other to another heir? Could I have seen that, and prevented it somehow?

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How do you deal with vassal inheritance warning?

You can simply preemptively revoke the title (watch for the relations penalty) or you can try to murder the heirs, until they're from your realm (if that's possible).

How do you not lose land when vassal dies in ck3?

Keep the stuff in the de jure capital area and give every non-primary heir something, and never, ever, have multiple top tier titles (and with the confederate partition don't even have the potential to create a second top-tier title as it will do it for you on death).

How do you avoid losing titles on succession ck3?

Raising crown authority requires you to spend prestige, so you'll have to work on gaining more of that if you can't afford the cost. You can also only move up or down one level at a time, and each time you change it, your vassals will change their opinions of you.

[CK3] 5 Ways to remove troublesome vassals

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Images: Mitchell Luo, ROMAN ODINTSOV, Ron Lach, Ron Lach