Why did my player heir change from my grandson to one of my other sons after reforming my faith?

With the Northern Lord Flavor pack, I'm playing a High King of the North Sea. My current player heir is my grandson, the first and only son of my second son.

My first and second sons are both dead. I have a few remaining living sons who are alive and well and a few are already Kings in their own rights within the empire.

The whole idea of my current character was to reform the Ásatrú faith. I am listing only my changes (no tenets modified):

• Main Doctrines:
  • Clerical Tradition: Lay Clergy
  • Head of Faith: Temporal
  • Religious Attitude: Fundamentalist
• Marriage Doctrine:
  • Consanguinity: Avunculate Marriage
  • Kinslaying: Shunned
• Clerical Function:
  • Control
• Clergy Doctrine: Temporal, Revocable

Once I hit the reform the faith button, my grandson is no longer my heir. In the realm view under Succession, he isn't even listed as an heir. Only my five sons receive titles in that view.

What even more odd to me, my third son already received a kingdom granted by my high king, namely the Kingdom of Frisia. (I also conquered his claim on Estonia though that shouldn't count to his inheritance.)

Since Frisia should count to his inheritance, he shouldn't and doesn't appear in the succession view at all before reforming the faith. After reforming the faith, he suddenly he gets all my primary titles. That doesn't seem in line with the idea of confederate partition.

I am really confused what was going on here. Did I miss something about the implications of reforming my faith or is this a bug?

I also checked that my grandson changed his religion to the reformed faith since I have a Same Faith law active. He did.

The only think that I can think of is that his claim to inherit failed because my second son could not change his religion because he's dead. That would stop my second son from inheriting, and my grandson cannot inherit what my second son could never inherit. Yet that seems like a broken game mechanic :D

Line of succession before reforming my faith:

Afterwards:

The issue stems from the

• Main Doctrines:
  • Clerical Tradition: Lay Clergy
  • Head of Faith: Temporal

change and is most likely a bug. The inheritance broke pretty much due to the fact that your second son could not change his religion because ... he's dead. And since only people of the same faith can inherit, his line is completely removed from the succession.

This is strongly coupled with the newly created title Ásatrú Fylkirate that has its own inheritance rules. If you were to destroy that title, your succession would fix itself (but you don't want to do that).

There's no known fix for ironman games.

For non-ironman games there can be workarounds – the best one I found was using a mod:

You can designate an heir using Daddy Pika's Cheat Menu.