Playing as the emperor of Britain one Duchy keeps breaking off

[Slaists]

I am playing as a Welsh emperor of Britain, holding the crowns of Wales and England; Ireland and Scotland is ruled by my vassal kings.

One English Duchy (Guenet) keeps breaking independent every now and then for some reason. The inheritance outside my realm is not allowed (in Britain, Wales and England). The Duchy does not become a vassal of anyone else either: just shows up as an independent Duchy in the middle of England. When I offer vassalization, they immediately accept (hugely boosting my bad-boy score, which is the annoying part).

Any ideas why this would keep happening?

Unfortunately, this is an ironman game, so I cannot go back to an earlier save.

 

[Castimirr]

I have had that happen before but it has never been consistent enough for me to figure out what is going on. I'm not over my vassal limit. I mostly notice it when I load a game but I don't know if it is directly related.

 

[Kdapt-Preacher]

I was just about to make a thread about this! I'm playing as Byzantium, and multiple times now I've looked over at Greece or Anatolia and found that one of my viceroyal dukes has become independent with no obvious cause. I haven't died, the duke hasn't died, there hasn't been a war, I certainly haven't granted independence. This has happened about three times in 50 years.

EDIT: Image isn't working, but it's here: https://imgur.com/a/fDgxzgL
You can see that this is the same guy I granted it to, less than ten years ago, and he doesn't have any opinion modifiers relating to either independence or a war. If anybody can figure out how to stop this, I'd love to hear it, since I don't even have a CB to vassalize this guy again--I'm going to have to fight for each of his counties separately.

 

[Naughtius Maximus]

I have find it usually happens when my vassals have revolts. It's the rebellious vassals under them that go independent.

Maybe the trigger is when the top liege dies while this revolt is going on.

It's just speculation though.

 

[Kdapt-Preacher]

I have find it usually happens when my vassals have revolts. It's the rebellious vassals under them that go independent.

Maybe the trigger is when the top liege dies while this revolt is going on.

It's just speculation though.

That definitely isn't it--you can see in the screenshot I posted that I'm the same character that granted the viceroyalty in the first place.
OK, I went back a couple of saves to figure out the sequence of events. Anselm here was the Count of Arigh in Africa, and I granted him the Viceroyal Duchy of Thessalonika, making him my direct vassal. I'm his de jure liege as Byzantine Emperor, but the Kingdom of Greece does not exist. My viceroyal king of Africa went to war against Anselm to take possession of Arigh in his capacity as Duke of Mzab, and won. This presumably led Anselm to emergency usurp one of the counties in Thessalonika, as it should have, but it seems to have also made him independent. At no point before independence did his liege change; he was my direct vassal the whole time. I did not get any kind of notification when he became independent.

 

[Slaists]

I have find it usually happens when my vassals have revolts. It's the rebellious vassals under them that go independent.

Maybe the trigger is when the top liege dies while this revolt is going on.

It's just speculation though.

I suspect, this might be close to the mark in my case. My vassal king of Ireland was at war with the duchess of Guenet who held a county in Ireland. The Irish king won the war and his county. The duchess was not his vassal but became independent (of the empire of Britain and the kingdom of England)...

So, I suspect, the county in Ireland was the duchesses only holding. Once she lost the war, she auto-usurped some county in Guenet becoming independent in the process. This independence thing does not make much sense though (especially as revassalizing such x-vassals costs a vast amount of bad-boy points). Probably should be posted to the bugs?