Army of Two: The 40th Day's campaign DLC, Chapters of Deceit? Well, according to Eurogamer, EA is well aware of the issue regarding faulty Achievement and Trophy unlocks and is currently working on a fix.
"There is a known issue affecting Achievements in an Army of Two DLC pack," an EA spokesperson said. "We are looking into a solution and hope to provide an update soon." In the meantime, we suggest you enjoy one of those other games that inspired The 40th Day. It's not a bad way to pass the time until you wait for "soon" to come around.