I had a similar one but with Battania taking over, it seems like once a kingdom has an army with 1k soldiers it just destroys everyone.
Personally I started a new game, as I also gave my banner to the king of the Sturgians and the Sturgians disappeared a week after.
There's still a battle going there between Battania and the Southern Empire, but overall it seems like Battania has over 14k power against everyone elses 2k
Started a new game, this time around I'm joining the Vladians at first and then creating my own kingdom, thus far I'm ~10 hours into the game and everything looks normal.
For what I believe, the vassals leave the kingdom they're a part of way too easily, you can read a kingdom with a lot of fiefs and doing pretty well losing a clan every week until they just disappear, I'm not sure on why do they just leave their kingdom, but they do, and those are forces that appear, seemingly out of nowhere, with 95 soldiers in the map a second after and in a different faction.
The balance to me is keeping vassals from leaving countries for the first year of the game, then limiting early armies to ~600 soldiers so that they can siege one or two cities but can't really steamroll all over the continent. Either that or buff castles during a siege, which would also make it way harder for the player to actually get a castle to start their own kingdom, mostly as you usually get gang fricked by at least one army during your first siege.