Tryndamere Undying Rage (Doesn't Always work?)

In the past couple days, I've started playing again after a fall hiatus. I've noticed there were instances where Tryndamere's Undying Rage did not protect from death. The interval after activation where death occurred generally was between 2 and 4.5 seconds, and I cannot find a direct cause for most instances, excluding Teemo's poison, which was on the underside of 2 seconds after activation. Other champions (most recently) where death occurred was against Fiora, where I have little hope of finding a true cause; my best guess would be the splash damage from Ravenous Hydra, but I am skeptical of that being the source.

If a recent update caused Tryndamere's Undying Rage to be less of a full protection against death, I apologize for wasting time, but this has started to become a serious issue in my play style; I am becoming increasingly frustrated and making reckless moves I would not normally make. I would appreciate a reply/address soon.

The interval where this issue takes place is somewhere between rare and uncommon; However, if recent matches are any indication, that statistic is slowly increasing.