When you reorder units within your stack you must move at least one square for it to take effect.
For example, lets say I have 3 Lt Calv within my stack. I can deselect and select them to reorder the individual cavalry so that the ones with the fewest moves are up front for battle. However until I actually move 1 square that re-order doesn't take effect and if I attack a city (because I am right next to it) the stack uses the order when it was first grouped.
This is a minor but annoying bug because often I have multiple units of the same type and one has 1-2 moves left while the rest have a lot and I want to be able to move the survivors as a full stack. Instead I have to drop a unit behind because the game messes up the fight order.
It would be best if the game simply 'auto-reordered' duplicate units for you so that the fewest moves was always up front.
KGB