Tryndamere ult's delay

I am main trynd player, got about 800 games or even more. I am sad because of bug that assist me the whole games i have played with this champ. Its similar to the old bug of renekton and nasus. It looks like u press R in the right moment but it doesn't trigger, it has something like 0.5 sec delay - but in the description of his R is no such a thing like this. Please Riot check it. U can easily check it on Darius vs Trynd when Darius is dunking and trynd cant ult in this moment.

"ACTIVE: Tryndamere **instantly **gains Fury and becomes immune to death for 5 seconds, during which his health cannot fall below 1. Undying Rage can be used even when affected by crowd control."