The enchanting quest Controlling the Elements is bugged at the moment, the Remnant of Water item does not drop from the end of dungeon chest off Ozumat. Looking through various threads and Wowhead comments it looks like it has been working off and on throughout the years.
I would assume the recent conversion to M+ might be the culprit this time.
For the record, Normal and Heroic are currently both bugged and not dropping loot at all. M+2 dropped a chest however the item did not drop from the chest and it is a 100% drop chance item.