Originally Posted by MidnightExpres
right my friend have only the SP achievements in this Game and we play coop on this 2 new missions and he get all 8 achievements from the DLC! a other friend have only 2 MP achievements (create a mask, invite a friend) and get only 6 achievemts on this DLC! he get no Dr Wu and the mask achievement on the DLC
I understand, in fact that actually makes a lot of sense as I just tried my secondary account, and it began to unlock the achievements normally. It has never played this game before so I think you are right.
You may have found a rather big piece to this puzzle. Good job mate!