Hey, just to help everyone out...what I've started doing, and I haven't had a problem since, is whenever you lose a game, relog. You don't have to drop the quest or anything, just relog after every loss. If you win the game and get credit, you will get credit for the next game if it's a win. If it's a loss, relog, or the next win may not count.
I can't say this is a fix and I can't say it has any real effect on the problem, I just know for a while now I have been having this issue, but since I started relogging after every loss, I haven't had it happen to me or my group since. Hope this helps someone.