Why did my vassal become independent?

• So I lured the heir to Kingdom A into my court.
• I gave him a county and a Kingdom.
• Then I killed his father.
• He got the new Kingdom… and became independent.

In CKII I'm pretty sure that since this wasn't a "promotion", he should have remained my vassal… is this mechanic in CK3 different? How?

PS: I'm Emperor OFC.

Solution 1:

This trick used to work very well in CKII. But in CKIII it only works if the new title of your vassal means that they stay/become your de-jure vassal. Which means you need to personally own a title which their new title is a de-jure part of.

For example, if you are the king of England, then you could pull this off to gain control of the duchy of York (because it belongs to England), but not the duchy of Powys (because it belongs to Wales).