Thanks a lot for the feedback!
I'm currently reading through MarkShot's different AARs on CotA and interestingly, he also speaks about my question. From reading this, it seems that a unit SHOULD do something reasonable, i.e. at least change the formation instead of running full-speed in column formation into the enemy. But then again, I understand that it depends on a number of factors.
One question that comes up in connection therewith is the question, how much information local commanders actually have.
Thinking the core concept of the series further, there should probably also be a concept of information delay and/or deficit: While I, as the theater commander may have received information about an enemy unit from Unit A, which is located in the path of Unit B, Unit B may have not received this information (yet) and could not react. Thinking this further, I actually may not know where my own units are (either because communication is interrupted or because their own assessment of their position is wrong.
I understand that this would be difficult to implement in a meaningful way (and probably very frustrating to play) but interesting nevertheless. I could envision a system where I "send off" my battalions with orders and they would basically "fade out" on a map when I have incomplete information and then refresh from time to time once I receive information. Not sure if this would be fun, though ...