This isn't the only issue with the Fire Shield. It has another logically contradictory aspect. For example, when using the Vulnerability Medal, units immune to fire retain their immunity to this spell, but units immune to fourth-level magic lose it... Something doesn't seem right... :)
I'm certainly no expert in these matters, but I'm just saying... how can fire defeat fire? It's like trying to wash water away with water... there will only be more of it... **** doesn't stick to ****... Here, it seems to me that it's not some kind of magical immunity (for example), but rather a natural one, because they are made of the same material that they are attacking.