They have to fix the Exfil & Disarm success at the same time. I played a couple games where we were waiting for the chopper and Pred arrived and we downed him and defused the bomb.
So then we had to protect the body and then the chopper arrived and we stayed to complete the body objective and the chopper left and ended the game as Left Behind. The protect the body mission should overlap the Exfil i would think.