"Install by faction" in your realm to a "ruler" outside your realm shouldn't be allowed with out a Civil war.

[Omaew]

In my recent 867 start Italia empire playthrough.The spanish muslims managed to conquer almost entire Iberian and established their own Empire.
I decided to declare war for Andulasia sultanate. After winning,I distributed the lands within my dynasty (Norse reformed astrau)but still had not usurped the kingdon title
After a while,a notice popped up say One of my alliance is not longer valid since he's no longer a ruler.
I clicked to check and turn out the emperor I just fought just got installed to Duchy of Cordoba by Faction demand. Meaning, I just suddenly lost some lands w/o a war while the truce is still on even.
I normally don't mind faction demand "in" my realm regardless of some random claimant from different religions that much coz my land is still mine. But if the faction want to install "a ruler" with lands outside my empire it should have become a civil war.

 

[fodazd]

I agree with this. There should at the very least be a warning that I am about to lose land because one of my vassals is fighting a faction pushing for an external claimant. I should also be allowed to join this war on the side of my vassal.

 

[nightcarc]

I'll second this one as well. A while back I had a run as Spain where I was starting to try and reform the Roman Empire, taking over bits and pieces as I could. It turned out that one of my cousins who I had granted Egyptian lands to had claims on the Kingdom of Georgia, so I declared war on the Byzantines for the land and won. A little while later, however, the my cousin's Georgian vassals pushed through a faction demand to reseat the deposed king, who then not only put the Georgian lands back under Byzantine rule, but also took with him my Egyptian lands as well since my cousin was now a vassal of the Georgian king. As a result, not only did I lose a bunch of land (more than I had even gained by taking Georgia in the first place), but I also had no way to retake the lands because the only claimant was no longer in my realm. It really makes no sense as to why a ruler would just sit by and let a large chunk of their nation just switch sides--I could have easily fought off the faction on my own, but was never given the opportunity/ability, and just had to pan over at some point to find I had randomly lost a good portion of my eastern lands.

Picture showing the outlined lands and the (new) king under Byzantine rule:

History of the title showing my cousin (Doux of Delta) who lost the title due to faction demand:

 

[kgptzac]

Faction demand causes higher liege to lose land from realm sounds exactly like treason to me. Top liege should be in the war on defender's side and all attackers should be flagged as criminals.