During war, players could easily ignore the grievances rules and just ransack administrative centres of the cities, what makes the idea of grievance system and the protection of the city more meaningless. So the solution seems to be simple to implement:

1) City centres and administrative centres with occupied status couldn’t be ransacked.
2) Administrative centres ATTACHED to the cities of the foreign empires couldn’t be ransacked even during war

So that will save players cities from mindless ransacking, and force players to play the rules of the war and grievance systems. As you can see, after you win the war and get the city you could easily ransack it and do what you want with it. But while the war goes on, the city centre must be protected at any cost. Btw you still can ransack non protected districts, but you can’t “deattach” outpost by its ransacking. Also player shall still be able to bombard/nuclear the administrative centres.