Warlock tanks and Nether protection

Not really a thing for bugtracker,
but over years I noticed common bug on unofficial TBC servers that warlock destruction talent, Nether Protection, sometimes went terribly terribly bad - when it *poped* making you immune to fire attacks, mob targeting you suddenly lost interest in you and went for someone else. Which in case of being warlock tank in raid is... pretty bad :D
So is it working properly? Making you immune but mob still keeps his aggro on you? Too low lvl to test myself.
cheers.

Ull be able to test this in 2 days when ptr goes in phase 2 with instant 70 chars.

Nether Protection is supposed to make you drop threat. Besides, Warlocks tank as Demonology.