I am playing as a Jainist character in south of India, where I created a kingdom title with primogeniture succession. Jainist allows you to select your heir so whether it is primogeniture or ultimogeniture or whatever does not really strictly matter.
However, interesting things started happening once I inherited three kingdoms (by marrying a neighbouring queen and designating our child as the heir). She had a mixture of succession laws, some were primogeniture and some were gavelkind.
Now I thought if I change succession in all gavelkind kingdoms to ultimogeniture, my selected heir would become the heir of every kingdom. However, unexpectedly, the actual youngest child is now the heir of ultimogeniture kingdoms, while the selected heir is only the heir of kingdoms with primogeniture...
Can someone shed some light on the mechanics of Indian succession with mixed succession laws?