Please explain me this succession

[El Greco]

I am the King of Italy, married to the Duchess of Silesia. I have a son, Angelo, heir to the Kingdom of Italy, married to the Duchess of Epirus. My son has 4 daughters (all 4, including two twins, named Giulia... wtf...) and a son (my grandson), Callisto, born 3rd after the two first Giulias, who is the heir to the Duchy of Epirus.

The Duchy of Silesia succession law is Agnatic-Cognatic Primogeniture.

I would expect that my son, Angelo, would be the heir of the Duchy of Silesia. But no. The heiress to the Duchy of Silesia is Giulia, my grand-daughter; second in line is her sister Giulia. Now I would expect that at least Callisto, my grandson, would be third in line, but no; 3rd in line of succession for the duchy of Silesia is my 4th granddaughter Giulia...

WTF is going on here.

Version is 1.04, no mods.

[pumapaul]

There was a bug with the inheritance switching around nonsensically in 1.04.
That's probably what's going on here. Your son should be the heir to everything, then his first son and so on.