Currently there are a couple of bugs with formation auras, i.e. with Athens' Iphicrates (since he's the only unit that has one).
The easiest way to reproduce the bugs is to have a group of units with Iphicrates and a single unit far away. Select them all, select a formation (the group will line up), and click to have them move somewhere. When the single unit joins the group they'll combine and line up in a new formation. But you'll notice that (1) the single unit doesn't have the +3 armor bonus like the other units and (2) when you remove a unit from the group or change the formation to "None", the units keep their armor bonus. (When units are promoted they lose the bonus, so there's some micromanagement involved in taking advantage of this bug.)
It seems both bugs are from rP14483, when "twin formations" were introduced.
- To fix (1), when adding members we need to apply the current auras to new members and then apply the new auras to all members.
- To fix (2), when joining two formations we need to remove members from their old formation (remove the auras) before adding them to the new formation.