No it is fine.
Same as elemental.
If you hit a fire elemental with fire damage he is healed (same for all elementals or any other ennemies).
Imarion
It's fine for the NPCs in that it teaches players about elemental interactions during combat and keeps players on their toes. It's not fine in that it works the same ways for players and creates trivial encounters where players are being healed by enemy attacks rather than taking damage.
For players it should just be a resistance to the element (and immunity should never be achievable).
