When finishing a Monolith of Fate mission, you get transported to Echo of the world to open a reward chest and launch a new mission.
The chest contains a possible arena key, it is possible to just town portal back and forth to keep re-opening this chest in order to get arena keys (and a few easy reward drops).
How to re-create bug:
Finish a mission, loot chest
Press T and go through the town portal
Use portal again to go back and loot chest again
Repeat as much as you like.