I see this bug at least every other sPvP match with at least 1 opponent. For whatever reason after downing an opponent they cannot be hit or finished. There is no blindness or anything of the sort affecting the player attempting to kill, and when the player attempts to finish the downed opponent, the animation goes all the way through even to the finisher, but it doesn't affect the downed opponent. This is extremely frustrating to have to deal with, especially if the person is downed on a point. I suspect it has something to do with them getting stuck in the terrain (as it's often on stairs or near a ledge, but I can't confirm.