I've noticed this too but I assumed it was because curse may work on the monsters like it does the players. When a player gets cursed, not every attack and vial they have gets the curse status put on it. So if the weapon you're using is cursed, you simply change to one that isn't and continue attacking. I assumed that some enemies weren't ALWAYS taking damage everytime they attacked because maybe only one attack of theirs is cursed, because it does seem like enemies take damage when they do one attack but not another.
I've noticed this too but I assumed it was because curse may work on the monsters like it does the players. When a player gets cursed, not every attack and vial they have gets the curse status put on it. So if the weapon you're using is cursed, you simply change to one that isn't and continue attacking. I assumed that some enemies weren't ALWAYS taking damage everytime they attacked because maybe only one attack of theirs is cursed, because it does seem like enemies take damage when they do one attack but not another.