Bug in Zed's passive

Zed's passive is described as dealing extra magical damage to enemy below 50% health, this works correctly to minions and neutral monsters, but when it comes to enemy champions it does not. It seems Zed's passive can only works on enemy champions when they are below about 35% or lower health, not exactly the described 50%. I have played Zed quite often recently and have met this bug constantly, and I DO HAVE TESTED THIS IN A CUSTOM, it is quite the same, not working as described. Pls fix this bug asap, thanks.