This morning, another bug was found by MTG Arena players
Some players reported that a few quests had their progress stuck on step 19/20, and the gold reward was being granted multiple times.
Image Credits: Wizards of the Coast
Cards Realm
Many players abused the bug by entering matches multiple times to cast a single spell to complete the quest and then concede the game. This allowed them to receive the gold reward over and over again.
Image Credits: Wizards of the Coast
Cards Realm
Apparently, the bug has already been fixed and you can no longer receive the rewards of a quest multiple times in the same day.