GAMEPLAY - Tryndamere still able to use ultimate after Garen's Q(silence)+R(Ultimate) combo

I have been playing against Tyrndamere top lane with Garen a few games now and every time I'd get him low enough to finish him off with Q and R combo he could still use his ultimate even when he's silence. I have always made sure the combo with kill him with his remaining HP and recently even with the Villian passive on it still doesn't kill him

Reproduce issue

1. Get Tryndamere to low HP enough (around 300 - 400)
2. Cast Garen's Q on Tryndamere to silence
3. Immediately follow up with Garen's R