There is currently a bug where dropping a city territory when an attack has been launched causes the city territory to become unclaimed instead of going to the attackers, essentially cancelling out the attack.
We are aware of the guild that was affected by this on Sunday and will refund the amount of silver cost to launch the attack. NOTE: Any exploitation of this bug will not be tolerated and we will take appropriate action on this.
Our developers are currently working on a hotfix to resolve the issue. We will update you as soon as we receive any new information. We apologize for the inconvenience caused.
– Mytherceria & Dev Team