Oha, I see. I never noticed this behaviour before. It's a strange game mechanic, but I suppose it was the best they found to keep the player faction from snowballing to victory. I did notice wars progressed more normally when I reduced campaign AI difficulty.
But it would be nice if 80% of the lords didn't desert a siege before it's done and then everyone leaves anyway, whether I'm there or not. That's definitely broken campaign AI. But good to know it's the game, not just Pendor. I think I'll play with easier campaign AI now, that seems to actually work.