Hello,
I do not know if this has been reported before, and it is probably not that significant of a bug, but I noticed a reoccurring issue as I was running the Mechanar, on both heroic and normal with my friends. If the group leader sets the loot to FFA on Gatewatcher Gyro-Kill and then sets loot back to group loot to continue the dungeon when the Cache of the Legion is looted it will be set to FFA loot. I have not attempted changing loot on any other bosses to see if this will trigger the chest, and I can only confirm that if loot is on FFA for Gatewatcher Gyro-Kill the Cache will be set to FFA.
Thanks for reading,