Happened to me in the past too. Ran a dungeon, only to not get any reward from it. Got exp from the monsters themselves, but none for finishing the dungeon.

I dont know what causes it. I just know it did, it never happened again for me since, but i suppose they may have re-used a piece of old code during the latest patch and revived this problem.

Do hope they look into what might cause it. I'd hate for it to happen again