The event "Defeat the Ley-Line Anomaly to disperse its destructive energy before it overloads" usually awards a Bonus chest that contains an Exotic Gilded Strongbox each time the event is completed; and once a day per account the bonus chest will also contain 1 Mystic Coin and 1 Crystallized Ley-Energy.
I usually do the event the first spawn after reset, in Timberline Falls, as I did today.
I wasn't awarded a Bonus chest though, and thus not the desired Mystic Coin.
This also wasn't the first time this happened to me. And when I go to an additional later spawn during the day, I get only the Bonus chest without the Once Per Day And Account items.
I got an XP/gold reward for the event itself (Gold tier even). I was participating with "Kaylee Solanum".
On a previous occasion the bonus chest was lacking when participating with "Xanxio Nightingale". Both of these characters have gotten the bonus chest usually, just sometimes not, and I wonder why that is.