This has happened to me twice now. I thought the first time was a fluke but apparently its an actual bug - just difficult to reproduce.
When you're on a quest and you have a defeat a boss - if you defeat the boss with a damage per round spell and the final round of damage is the exact amount of hit points the boss has then the end of battle is triggered and you are free to roam but you don't get quest credit for the victory causing you to have to redo the fight.
You also don't get item rewards - so the quest part might not really be a significant factor.
A hearty thank you from our development team. Your very clear report allowed us to pinpoint the issue in minutes! We will make sure this is fixed in our next content update to the game.
This has happened to me twice now. I thought the first time was a fluke but apparently its an actual bug - just difficult to reproduce.
When you're on a quest and you have a defeat a boss - if you defeat the boss with a damage per round spell and the final round of damage is the exact amount of hit points the boss has then the end of battle is triggered and you are free to roam but you don't get quest credit for the victory causing you to have to redo the fight.
You also don't get item rewards - so the quest part might not really be a significant factor.
The quest credit is definitely a bug, but for the no rewards/drops thing, I've learned that if this happens, just feed your pet an item and you'll get some of the items you would've gotten from the actual fight if that bug didn't occur.