Hi. I've seen this in my own campaign vs AI and also just seen it happen to Lionheart in his Lokhir campaign (part 13 38mins).


When a settlement is sacked the buildings aren't being damaged or downgraded. I sacked a kugath settlement (his last and a province capital) which was at L5 but it was undamaged. 


In Lionheart's campaign skaven sacked him but again no damage. Is this a bug?