Not a bug.
The monsters and daimons are simply very resilient to being killed.
The undead will be wiped out in a single attack, but the monsters and daimons will remain for many turns with each battle merely wounding them rather than killing them. So they come back and attack again before the walls can be repaired.
This is not a bug. It is simply a matter of taking the capabilities of certain units to soak up damage without actually dying and turning them into a strategic asset to keep the human realms from performing repairs to their walls.
This has been seen and remarked upon by players in Dwilight where the monsters and undead are simply swarming the cities one turn after another and wearing the walls down without any chance to repair them.
This is more a feature I think than it is a bug. Some have called it an exploit. It is certainly possible that there will be a decision to change it, but that would only happen if those in a position to do so recognize that it is a widespread tactic across multiple realms, and even continents. And the reactions of every human realm (AKA players of the game) targeted by it are universally negative based on what I have seen.
Calling it a snide comment to note this problem is attempting to deflect the issue and suggest it is not real. I assure you, based on what I have seen both in game and on the forums, this is an issue that the players are very much aware of and are universally unhappy with.
