Cho'Gath R cooldown reset [Gameplay]

When feasting on a minion and killing it you are supposed to get half the cd back. But many times in my last 3 games my ult has gone on full cool down when eating monsters and minions. EDIT: _The recreation was done in a customgame with only me no AI, the first time i noticed the bug was around 11:00 PM in a ranked team 5 v 5 game. _ I am not certain if this affects the bonuses vs champions but when you feast right before your auto attack would have struck the target the cool down doesn't reset, i've managed to recreate the ''bug'' on every monster and minion except Gromp, Baron Nashor, Dragon, Rift Herald. It might be possible to do it on Gromp and Dragon but i simlply didnt manage to do it in the coustom game i was in, and i couldnt solo either Baron or Herald to try the bug.