EU4: Why is my personal union ending?

I am playing as Aragon and I have a personal union with Castile. My ruler is called "Alfons VI de Trastámara". Everything is well, Castile's opinion of me is 200, and I have just started to integrate them.

However, now I got a popup saying that my Personal Union ended because "Alfons VI de Trastámara of Castile has died". And I get the restoration of union casus belli. But when I go to my court tab, I can see that King Alfons VI de Trastámara is still alive and well.

So, why did I get a message saying that the king of Castile has died, when we're in a personal union? My king is ruling both nations and is still alive. Is this a bug?

I am playing the 1.23 Persia patch with the Cradle of Civilization DLC installed.

Solution 1:

It turns out that their capital was besieged by rebels, and Castile accepted their demands, overthrowing my as their king. The text in the popup just didn't make a lot of sense.