http://www.wowhead.com/mission=1279/...fears#comments
read the comments on the link you posted.
"By XxTimetoKillxX (4,967 – 14·107) 2 hr 49 min ago (Patch 7.0.3)
If you have the quest on your order hall board, and kill the boss on the difficulty it requires without completing the mission first, the mission will disappear and be lost. Uncertain if this is intended.
Had a mission to kill heroic Il'gynoth but I couldn't have completed it before raid so I just left it - came back after raid and it was gone."