Associate
Not sure if it's a bug or something not working correctly but just played a game as Persia and the Immortal wouldn't melee, despite still being tagged as a melee unit and having the melee unit promotion tree. Annoyingly, it meant I couldn't take cities with it and it ended up being an expensive (and very slight) upgrade to the Archer.
Seems doubly weird as you can upgrade the Warrior to it which is going melee unit to ranged.
You can guarantee it will be a bug Civ games have a lot of them