I think that's intended since your fighting two different enemies it makes sense that you have to hit both atleast once to win the fight.I just tested it. When you kill one of them without attacking the second, the battle continues until you finish off the second one, even if they shared HP.
@Sekibanki ,what talents did you use when you defeated the faes? I'm thinking Burning Weapon may be a culprit, but I can't tell from the screenshot!