Feast bug

Ive been playing for a few games straight in a row and I noticed something about his feast (ult.) When his ult got changed feasting a minion would give 50% mana you would have 50% less cd, that's what supposed to happen right; but sometimes whether feasting a minion in lane or a monster in the jungle I would get full cd on my ult, I still would get a stack on my ultimate but i wont have it up and a while. This frustrates me because i'm able to feast less frequently when i'm pushing waves or stacking up in the jungle

rito pls.