Targon Brace Tooltip and maybe bug (Tried only on Rakan support)

Hi, so I'm reporting something weird that's been going on a few games as I play Rakan support with Targon Brace. The tooltip at level 1 says anything below 270 hp u can execute for gold and 15 health, but sometimes even with a stack this does not proc and the minion doesn't use the stack. You can watch my recent 2 games to see this, as this got my adc very mad even though I was attacking the minion below 270 health for the proc it just didn't happen and the minion was killed by another minion. Please tell me if this is me, or an actual bug or maybe a tooltip error.