Rift Herald can be a bit too easy

About the backstab on the Rift Herald, it is said that ¨The eye on Rift Herald's back periodically opens, causing basic attacks from behind to deal 15% of its maximum health as true damage¨ and that ¨If Backstabbing Rift Herald brings it to 15% health or below, the eye stays open.¨. But, during a game I played today, the first time we killed it, it acted normally. On the second time though, I was backstabbing him, and every auto attack I did on him dealt the true damage, from the beginning to the end. I was playing as Zac and I was helped by Talon to defeat it. Hoping this helps in someway or another, if it ever happens again, I'll add it on this post.