Loot missing?

I have been doing the ques Sanctum of Domination - Damned If You Don’t where you need to kill Kel’thuzad 4 times to get the skip in the raid, two days ago I killed him for the fourth time, but he didn’t drop the phylactery shard like the other three times(last time I killed him was about 3 weeks ago)
I open a ticket to see what happen because is suppose to be guarantee loot but the response I got what that is not a 100% chance to drop the quest item
Is this true??? Or did I just get a bs response