Yap, same issue here. After a while clans yust decide to leave the kingdom for no reason at all.
Finished taking about half of the map and clans yust started leaving.
Relation at or close to 100, wealth rich or very rich, biggest army on the field, winning all wars despite my dumb vassals declaring wars on the entire world...nope, not good enough. They still leave the kingdom.
This needs to be fixed or the entire end game is pointless.
It looks like now the only way to take the whole map is to yust do it solo with only your clan and execute the rest of the world.