I'm not sure if it is bug. Definitly it is strange behavior.
When i defend city with army against army with catapult.
When i destroy the catapult my army battle strength is still counted as if the catapult had been alive. This is clearly mistake. How could catapult deal damage to walls when it is destroyed.
Note: In the case when catapult is in the middle or begging of fight order.
I tried to explain as best I could. If you have any question I will be happy to answer.