Yeah, I have also noticed this.
I love the fact that AI allies often actually help you in your wars, defend your territory and hunt down your rebels - I am Commonwealth and Bohemia/Hansa/Austria are ready to send their armies under Moscow in case of our often wars

- but when I control my troops on the frontline I am never sure if AI army standing next to me will help me or not. I guess it fails at calculating how big chances of success are and refuses to send troops to a 'lost' battle.
Honestly I would prefer if it behaved like
'if human player is near, ALWAYS help him in a big battle unless your combined troops are smaller than half of enemy army'.