I'm not part of the staff, but I think I might know a possible reason of your problem. Let me explain to you how this type of game works.
When you play the game, your computer calculates all of your movements and shows them without any delay on your monitor. At the same time, it sends your commands to the central server of League of Legends.
This server processes all movements of players and commando's given by them and passes it through to all players in the game.
Not the tricky part: This sending of information takes some time to travel, ofcourse, depending on the speed of your internet (and the server).
The delay will only be like 200 milliseconds or something (0.2 seconds) but before the server has processed the information, calculated the movements of everyone else and then has send it back to you, that might take like half a second, before the answer is ready.
Now your program will sometimes first register you pressing R, then sending it to the server, but at the same time recieving the event of a death blow from another player.
What it will do is prioritising the information recieved and count it as death for you.
In short, the server might (depending on internet speed and processing power available) make activation delays of about a second, i guess. The best thing to do is to pop your ultimate just a slight bit sooner then the ultimate point of zero health. Can't think of anything else.
I hope this answer is satisfying.