AI battlefield algorithm.

Yes, this is a mathematical justification for the AI's behavior on the battlefield. The problem is that it's not always possible to quickly consider, evaluate, and calculate the influence of all factors. The AI's behavior can be defined as the most probable, but that doesn't mean it will act exactly that way – it may act illogically from the player's point of view, but logically according to the algorithm programmed into it. In addition, there may also be a certain degree of randomness in the AI's actions in some situations. Therefore, it is not possible to describe the AI's behavior algorithm on the battlefield briefly and unambiguously (without analyzing the code).
However, I would like to note that the "brainless" undead behave as they should – they go to the nearest enemy, while living units may ignore flanking maneuvers and go directly to the main enemy forces.