Cho'Gath's ult occassionally not getting reduced cooldown

I've noticed this happening before that Cho'Gath will sometimes not get the reduced cooldown on his ult when eating a minion or monster. You get the stack but his ult goes on the full cooldown. I just played a game where this happened 3 times though. That can make it really frustrating as it takes longer to stack and can prevent your ult from being up for a fight. It's normally pretty rare and you may have to munch on a hundred minions before it happens, but it can be pretty frustrating when it does occur.