Gameplay bug, Graves ulti reset

I haven't been able to re-create the issue yet because I haven't been able to gather up enough people to test it. But during a ranked match I played on Graves, early in the game, we had a 3v3 in the bot lane with both teams junglers. The enemy support (Thresh) was almost dead and running away, I had hit 6 during the fight and fired my ult as he was running. Just after I fired my ult, but before it hit Thresh, their team ADC (Corki) killed me, but my ult still killed Thresh.

When I revived (only about 10 seconds later as this was very early in the match) I noticed my R was already off cooldown. I didn't try to use it, as it was a ranked match and I didn't want a bug to influence the outcome. It is possible that it was just a visual bug where the ability looked like it was off cooldown when it really wasn't. If I can get enough people to try and re-create the scenario I will update with whether or not I was able to reproduce the outcome.