[Patch 5.5] I found a serious bug! Deathcap's passive not working on Katarina!

I was playing a ranked game with Katarina and I built the new item (Luden's Echo ) first. But after I bought Deathcap , I found the green number after "increasing 30% ability power" was ZERO! I had purchased Luden's Echo , Zhonya's Hourglass , Sorcerer's Boots and Deathcap. My AP was 500+. I don't know if the total AP was correct, but there's definitely a displaying bug.