HEX item bug

The Hex item seems to prevent too much damage.

Lets say I am fizz. I W+Q into a cait with 100 health over the protection limit. MY combo will deal the 100 damage, trigger the shield, and stop there.

Now my two abilities are on Cool Down, only dealt 100 damage. Her shield remains a full 400 health amount.

I'm not sure if this is normal, but right now against fizz, this items acts as a nullify to any spell as long as her health is equal or bellow the protection amount of HEX + 400 health shield.

I'm pretty sure the item isn't suppose to work that way. If it is, damn that's is some broken item to have on an adc (it gives a descent amount of attack damage too ). Gives them protection from assassins to the infinity no matter how much burst damage they do.