Battle of Adwa not unlocking

[Somrael]

I have slain a Scottish swordsman unit in Africa (True Earth map, true start locations) with Oromo cavalry. Recently, I slew three Toa units on my continent (Persian Plateau, random Tiny map, Kupe settled capital on another continent), all with Oromo cavalry, and the achievement has not unlocked for me.

 

[robal1991]

Battle of Adwa
Defeat an Infantry unit whose capital is on another continent with an Oromo Cavalry.

 

[Equilin]

I assume "Infantry" refers to the modern infantry unit, rather than the melee unit class.