Been having the exact same problem. Vassals/lords leave way too easily, after mere days, after being given towns castles, or 1million gold, and I'm not talking about mercenary lords who you expect to leave. I have multiple playthroughs going currently and it doesn't matter if I'm the kindest butterfly of a ruler with no evil traits or actions, with positive reputations with lords and only like 1 enemy on the whole map. I lose vassals the same exact rate as if I'm playing a head lobbing psychopath with -100 relations with everyone, including my own lords. They all leave, take all the castles and towns I gave them, even though they have way more than me. No amount of save scumming seems to stop it either. Meanwhile other kingdoms with much lower ratings have a dozens of lords that don't magically switch over to you like they switch over to others. The single player game is unplayable for me until they fix this ridiculousness or explain how to mitigate the issue by gameplay. If it is some other factor like kingdom policies or whatever the game should communicate why the lords are leaving so you can at least try to fix it or understand it. Otherwise this comes off as just a huge game breaking bug. I'm kind of surprised I don't see this spammed in forums or reddit as a huge issue that needs fixing.