Mage Bug (well, I hope it is one)

That's really weird, vanilla minecraft doesn't allow projectiles to pass through anything with a solid hit box (grass, other shrubs don't have this so they can be shot through)

 

If they were standing directly against the iron bars, then its because the arrow that is spawned for the damage spell is spawned in front of the player, not directly on them, therefore being past the iron bars. The same thing can be done if you stand against two blocks diagonal from each other, and shoot at a 90 degree angle (facing the corner where the blocks meet). The damage spell will appear to go through the blocks, when in reality its being spawned beyond them. I believe that the damage spells spawn in front of the player because if the arrow spawned was directly on the player, then it would immediately hit the player who cast the spell, rather than shooting at the enemy.

Edit- This could very well not be the reason this happens, but its what I think.

 

Yeah, the arrow is spawned in front of you. This happens with iron bars, glass panes and fences. I believe it is also kickable.