While fighting Accursed npcs wielding a two-handed starmetal axe, Last Stand will not proc in time for the 95% damage reduction. It seems to be either a ping/latency or timer issue. The perk needs to take effect immediately when health drops below 50 percent no matter what the player fights.
It seems as though Accursed attacks with two-handed axes somehow queue up and hit the player in rapid succession too fast for Last Stand to proc.
I have not noticed this problem with any other weapon, only two-handed axes on the Accursed faction.