CK II - CKII - 3.2.1 - Subjugation casus belli not available against one specific realm

[Rebble]

One line summary of your issue
CKII - 3.2.1 - Subjugation casus belli not available against one specific realm

Game Version
3.2.1 AZCE

What expansions do you have installed?

The Reapers Due,
Conclave,
Horse Lords,
Way of Life,
Charlemagne,
The Old Gods,
Legacy of Rome,
Sword of Islam

Do you have mods enabled?
No

Please explain your issue is in as much detail as possible.
I am playing a nomad, starting with the Khagan of Coumans in the 1066 start. I went on my way and conquered quite a bit of land without any issue, and using my subjugation Casus Belli.
Than, my character died, and his son took the throne. But I am now faced with an issue.

My subjugation Casus Belli refreshed since it is usable once in a character's lifetime. I can, as intended, use it on pretty much anyone not nomad , no matter the size, including the Holy Roman Empire.
Everyone except one person, the King of Rus' de Kiev, and I can't understand the reason why.

As seen in the screenshots, I am able to go to war with Rus' de Kiev, but not using the subjugation Casus Belli (it should be higher than the top Casus Belli in the screenshot), so the issue is not with prisoners of war or similar restrictions . They are not nomad, not even from my religion. And since I'm able to use it on the HRE, I'm pretty sure I fulfill all the other requirements.

Steps to reproduce the issue.
1. Load the save game linked below
2. Try to declare a subjugation war against Rus' de Kiev

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[Karlington]

It looks as if he's already involved in a few wars. Are any of those subjugation wars? IIRC the Subjugation CB can't be used against someone who's already defending in another Subjugation war.

 

[Rebble]

You're right, one of his wars is a subjugation war. After some tests in another game, it indeed seems like the Subjugation CB can't be used if the target is already in a defensive subjugation war (at least if it targets him), so that's probably why I can't subjugate him.

However, I had not found this information anywhere before (not on the wiki and not when trying to research my issue). Maybe instead of making the CB invisible, adding a check along the lines of "Character is not already the target of a subjugation war" visible to the player would provide a better explanation as to why the player can't start this war.

 

[Karlington]

However, I had not found this information anywhere before (not on the wiki and not when trying to research my issue). Maybe instead of making the CB invisible, adding a check along the lines of "Character is not already the target of a subjugation war" visible to the player would provide a better explanation as to why the player can't start this war.

Agreed, just making the CB disappear as they've done right now is a beyond stupid design decision.