With Candaphon, when he gains an extra turn after killing all enemies with A2 at the end of a round, the extra turn is lost at the beginning of the next round : the "extra turn" message and the complete skill cooldown of A3 occur properly at the end of the round, but the one turn cooldown of A2 implied by that extra turn is lost at the beginning of the following round.

Not sure the same phenomenon occurs with other champions who may gain an extra turn, such as Relickeeper or Hordin.