At the begining of the battle the corps is deployed with the regiments grouped in brigades and divisions.But after a few maneuvers they start to spread and mix up.Is this a bug?
No, it's not a bug.
After a combat, some units run away and the AI tries to replace them with the "best" units.
The criteria are : minimal distance, type of units... so it's possible the Division/Brigade coherence is lost.
JMM