Blizzard has announced that a recent hotfix has corrected immunities on Boss adds in Molten Core for Season of Discovery's raid release. While most players won't be affected and the transition should be seamless, some groups that entered Molten Core right on release may still have the old immunities active; to fix this, players should soft reset their instance.
Molten Core and Onyxia's Lair Now Live
Players who zoned in during the first few minutes can expect to find boss adds that have unexpected immunities.
If you wish to, a soft reset of the instance should result in the non-boss enemies spawning correctly. Also, quickly swapping to a different Heat level and back to your original Heat level at the Hydraxian NPC near the entrance will reset the boss add immunities without needing an instance reset.
Thank you!.