Garen Ult Bug

Slightly before the 20 minute mark it shows me killing Katarina and Sona killing me. It was only a split second between kat's death and mine. I used my ult to kill katarina and the Vi on my team saw me do it as well. I noticed right after that my ult wasn't on cooldown even though i undoubtedly used it. It was strange and if there is a problem with Garen's ult like this i'd rather it be solved than exploited.