I observed it in my recent game. I am declaring on Ming (and I did 3 times already), Ming is a target. I won, I have waited for war exhaustion to reach 20, then signed peace. Instantly I see that unrest in provinces goes from 17 to -3. Like Ming is loosing all war exhaustion as soon as war ends. Is it a new mechanic or bug? I soon will declare yet another war so I should have some save to investigate.