Progress on killing 1000 Steam giants often resets

[Xeinok]

I don't have an easily recreated situation for this but for myself and several friends for some reason we have noticed that our x/1000 progress for killing the Steam Giant Civs resets itself quite often... making it pretty much impossible to get the 1000 kills necessary for the achievement (on top of the issues where they won't spawn in large numbers on Nightmare/Madness).