Tsuihou Reijou Vivi No Funtouki: Youjo Ni Natta Saikyou Madoushi, Koori No Koushaku Ni Hirowareru

Viola, the 19-year-old vice-captain of the Royal Court Mage Division, is unjustly accused and exiled— —in the form of a little girl! Picked up by Duke Feld, a man from an enemy nation known as the “Ice Duke,” she quickly gives the alias Vivi and begins plotting to use the duke’s power to regain her original form… But things don’t go quite as planned. (Sana Scans)
https://www.cmoa.jp/title/321240/