Yesterday I got 3 missions that I can complete in 1 match (killing 5 with assault rifles in versus, killing 5 with assault rifles and 1 kill with precision rifles). When I killed the 5th enemy with lancer, I got the pop-up notification that I completed 2 of them (and in the screen where you can see your points and your crew, I could see my 2 missions completed).
I had to wait for the next map to get a Longshot, and I got 1 headshot, but I got no notification (also, no notification on the points screen too). I did a 2nd headshot with Longshot, was the same.
So at the end of the match, I got the notification (while you wait for the next match).
Question is why some missions gave you instantly pop-up notifications and others not? Every time that this happens I prefer to finish the match, but I believe that you can still complete the missions if you leave the match.