Players on the official forums have brought a visibility bug to Blizzard's attention regarding and Miasma, making it impossible to see the player character when stacked.Image courtesy of DenisTRUFFAUT on YouTube and posters on the official forums. Thankfully, Blizzard are aware of this issue and are on the case — a fix will be coming during a patch later this month!
The team has a fix for this that will be coming in a larger patch later this month. Read more on wowhead.com