Cho'Gath Ultime bug (Gameplay)

When I play Cho'Gath, when I use my ultimate on a minion or a jungle monster, there are times that the ultimate has a full cooldown, rather than it being cut to half.

1. I level up to level 6
2. I upgrade my ultimate
3. I cast my ultimate on a minion
4. The cooldown on the ultimate is full, not reduced to 50%