I'm not sure when that started cause when I grind with him I usually don't pay attention to the screen but I think that started after I had activated the mastery Giant Slayer. Septimus Behead skill reset the cooldown of Holly Sword skill but most important gives him an extra turn.
I noticed that sometimes that doesn't happen and ok in the grinding you lose some seconds but in arena losing a turn is crucial to some fights!
As you can see in the first round even if it kills the opponent doesn't get the extra turn and that goes on to the second and third round also !!!