Blue Post
Original thread hosted on Blizzard's official forums.
Luminous Dust 119/120 with achievement Dust 'Em Off completed
View on Blizzard forums:
https://us.forums.blizzard.com/en/wow/t/luminous-dust-119120-with-achievement-dust-em-off-completed/2265729/35
The team is aware of this issue and are investigating a fix for those who are missing Luminous Dust. Thanks for the reports!