This is a general thing that you see in almost every movie and game;
Two armies facing each other in formation, clashing, and suddenly all formation breaks apart and everyone is fighting in duels. From what I know that normally never happened. Usually only the front lines of the armies would fight each other.
It seems Kingdom Come: Deliverance didn’t get that right unfortunately.
For example in this video:
They advance towards the enemy in formation, but when they clash with the enemy all formation breaks apart and everyone is fighting in duels.