I'm not sure why it happened.
Macedonian heavy cavalry charged in the rear by both gallic protected cav and protected heavy foot.
So, on the first rear hit, the macedonians drop a cohesion level (as per the rules in patch 1.2) and fight at a disdvantage.
Then the second hit, they drop another cohesion level to fragmented and again fight at a disadvantage.
Then, it disappeared from the map.
The Macedonians had no skulls to show damage before going into the combat.
I'm puzzled

P.S. Scenario was Invasion of Macedon.