This happenen quite often lately. I play hunter and use Feign Death - Drums of Panic combo quite often in Battlegrounds. Almost half of the time, I will succesfully cast it, the animation of “fear” will appear and the debuff will be shown on the enemy player, but they will keep attacking me regardless.
This has happened against, druid, rogues, shamans, basicly any class. I did not want to report at first because I thought may beI am missing they have imminuty buffs or something, but now I am absolutely sure it has nothing to do with it (also if it was, the animation would not show). It lands as it should, target has no immunity of anthing that prevents fear mechanics, but it does NOT cause them to lose their control, and they keep attacking / casting spells as usual.