Confirmed CK III - Heritage bug

[GodVIII]

Short summary of your issue
Heritage bug

Game Version
1.4.0

What OS are you playing on?
Windows

What platform are you using?
Steam

Do you have mods enabled?
No

Have you tried verifying your game files (Steam only)?
Yes

How much "pain" is this causing you?
9

Please explain the issue you experienced in the most condensed way possible
My empire has male only partition law. But some of the duchies in my realm suddenly have seniority succession law. So the oldest member of my house would inherit those duchies when it's current holder dies. Strangely my heir would still inherit my titels, but his heir is the oldest member again. This makes my safegame unplayable. Tried to change laws and created the empire due to this bug, didn't work/help.
Edit: I now know more: There are three vasalls that keep on changing their succession law to seniority. I changed to playing them, set it back to partition, a few minutes later they changed back to seniority. This shouldn't be possible should it? I have high crown authority.
Edit: It seems to be a problem with the duchies themselves. Duchy of Styria, Lower Bosnia and Syrmia always switch succession laws to seniority regardless of who is holding the title.

Please explain how to reproduce the issue
I don't know

Is there anything else you think could help us identify/replicate the issue?
I "recently" (around 30 ingame years) activated "Unite the west slavs", maybe that has something to do with it?

I have attached a save game
Yes

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File(s) attached

 

[Duke Corvus]

Hi, GodVIII!

Thanks for reporting this! It looks like the AI prefers the Seniority law, which is as designed and which you can bypass by changing their contract to Forced Partition. However, they should have a cooldown after the law is changed that makes it impossible to change it back right away. We've added that issue to our database and will look into it.

Cheers!