Hi Ace Jupiter,
I also recently ran into this issue, button completely gone and nothing to click to get it back, the only workaround I found was an odd one, found here on the forums:
Go to create a scenario/map (the scenario map editor) and once in the map creation screen simply add a trigger for the effect “Use Advanced Buttons". Then click to “test” your scenario on the menu…magic, the advanced combat formation buttons come back.