Tryndamere's Ultimate

I've been an avid player of Tryndamere since I've started playing this game 2 years ago. Different accounts I have him mastery 7. Let me tell you, his ult has a small bug. Sometimes when something that does damage to you, like a turret shot is flying towards you, if you ult before it does, sometimes the shot still hits and kills you. This has happened to where Trynd will shout, and die without his ult going on cooldown. I've attempted tower dives before, gotten the kill, and saved ult until the last shot was about to hit me, the shout goes off but then it hits me and I die anyways. Idk if maybe this is a lag issue on my part, but I would like you to look into it for me. Clarity would help me out here.