About 5 minutes after Molten Core opened, we deployed a hotfix to correct immunities on non-boss enemies. 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!