Edge Of Night not going on cd

I was playing a game as Kha'zix and had purchased the item Edge of Night. On the opposing team was a Karthus. When he would ult, I would channel the item. I would be mid channel as the damage would go off from Karthus. I didn't take damage from Karthus ult when I should, but I took damage from deathfire touch mastery. On top of that, the item would only go on a 5 second cd instead of the normal 45 seconds. I thought maybe the timer had bugged, but I was still able to activate it the item for the spell shield effect. The only time it went on the full 45 second cd was if the channel was completed before blocking a spell. I was also able to spam the item while dueling a Maoki to block multiple spells. I don't know if anyone else has had this issue.