A bug occured last night (23/09/2016) during a match where me and my team did a 60 team kill match. We complite the challenge but the archivement didn’t unlocked and we hadn’t recived credits or title “Coop-etizione” (italian). As you can clearly see on the screenshot: http://images.akamai.steamusercontent.com/ugc/439488775828662959/5AE7C4BA5076E4B14D3FE44F05A547CECB6F2D53/ we did the challenge
Report it to the gearbox support team via a ticket and they will unlock it for you. It is glitched across all platforms I believe.