Before I go and post something that probably might not even be a bug, to any players old from vanilla, is this behavior something that you recall? An ability for Molten Giants that is similar to Emperor Dagran Thaurissan's Avatar of Flame (makes them immune for about 10 seconds to all shadow damage and spells).
I've looked up some old guides and found this 2006 one, but I haven't seen any notes explaining an ability on Molten Giants having this sort of an effect, or even ability.
2006 guide link - http://www.ownedcore.com/forums/world-of-warcraft/world-of-warcraft-guides/323-molten-core-guide-pictures.html
Emperor Thaurissan's abilities - http://db.vanillagaming.org/?npc=9019#abilities
Molten Giant's abilities - http://db.vanillagaming.org/?npc=11658#abilities
I know that this goes on bugtracker, I just wasn't sure at all whether this was a bug or not, I came in here with the intention of reporting it there after some clarification about it from the community.