You have to force update the stats yourself by playing another Easter map (to be sure about getting the Cubelet, play a map that's needed for the Cubelet). Doesn't matter if you win or lose.Try to play an other easter map (any gamemode). And have some patience.. the challenges can take some time to give you the reward! But not to worry, you will get it!
@Ducky lock? I think the question has been answered clearly!