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.