Mutable Star Cleric Vestments gives 90% physical resistance

I believe that item is supposed to double your armor while you are at full ES to no bonus when no ES. But instead it seems to give infinite armour as long as you have any ES. I tested againts this guy that hit for around 1k. My small adjustments to armor did very little (from 1k to 7k), but when I equiped this item. I got hit only for 100 damage right away. No matter if my armor was 8k or 1600.