Just tested this again - if the damage doesn’t kill, he does get the extra turn. He doesn’t get an extra turn if he kills. This was supposed to have already been fixed in a previous patch when similar abilities that check something on the enemy which deal damage were fixed (Orc Veteran, Silver Drakon), but what is happening here is that the secondary condition for extra turn isn’t checked until after he has dealt damage. If the damage is lethal, the enemy is dead when the condition is checked and therefore cannot fulfill the condition (in this case, they will never have hunter’s mark, because they are dead). Pretty annoying since in most cases, you won’t want to throw a basic damaging spell blocking mana at the front of the party unless it does lethal damage and an extra turn.