Cho'Gath ulti cooldown doesn't get halved by eating minions. (Sometimes)

I was playing Cho last night and I got my lvl6. I started eating minions and I got my 6th stack. Everything was fine and CD of ulti was working well. A Darius killed me and after re-spawning I just noticed that my ulti's CD doesn't get halved by feasting a minion. (I ate 2 superminions) The CD time counter started at 40 seconds and instantly changed to 80. (I didn't have any CD reduction source) After some minutes (I don't know how), the bug get fixed and ulti was working well. This bug needs to get fixed Rito. (Sorry for bad English)