It's been happening ever since the "Pilzsken" event yesterday.
Best you can do for now is take a screen shot of the Mission Complete page after the fight is over and submit it to UR Support, and they will reimburse you for any prizes you should have gotten.
If you check the missions complete tab on the missions page, I think you should see your prize there. I've been told this was just a display issue and prizes were correctly distributed.
It happened to me as well, with the Lil Jey mission of Kreenk Ld. When I checked the missions tab it just said : "...you won and 5 mission points", also the mission was placed in the expired missions group.
Now it displays the correct reward, and it's in the right place so it should be fixed already.