Challenging Smite allows you to kill tryndamere when he is Ulting

So i was playing a game today, and something strange happened. I was versing Tryndamere in a dual and I had skirmishers sabre in my inventory, so my I was able to smite him. Tryndamere's ult allows him to become invulnerable for a period of time but i was able to kill him after smiting him. The bug has probably already been reported but it is for sure a game changing on and i wouldn't want to win a fight because of a bug. Thanks