There seems to be a bug whereby completing Wildlife Event quests via GBG rewards (and possibly via other methods that don't display the main city - I've noticed this on more than one occasion) doesn't bring up the little green tick as usual when you return to the city view, so you can't tell that the quest has been successfully achieved until you manually open the quest window and find the reward has been sitting there waiting to be collected.
For example, I had a quest to collect 2 million gold and went off and played the GBG, during the course of which I coincidentally won enough coins as battle rewards to fulfil the quest. I had no idea I had already done it because the quest didn't show up as ticked.
(This event also seems very unstable - it is constantly freezing up and then reloading in the mini-game, even though I have cleared the app cache...)