I have never witnessed any AI intervention to my deployment. I suspect that this is caused when some units are not contained in a certain group but all are selected when the command is given, this will cause rearrangement.
If all 20 units are grouped in a formation, that formation will be retained when you scroll ( r/click and drag ) their new destination. Within this group you can issue commands to certain units, for instance, a cav attack and provided they are still in the main group, scrolling a new destination will bring all units back to their original formation. Then again you could choose to have multiple groups and scroll each out individually. This does not take that much longer but it does allow some readjustment. Also, you can remove some units from the main group without altering the formation of the remaining units in the group.
Like you, I was quite put out when RTW was released and I thought CA had ruined the commands. This is not the case, they still exist only now they have been made easier. Do you remember how, in STW, you could only select a single unit, all units of same type or the entire group once you had formed a group? That was changed in MTW and you were able to issue commands to multiple units within a group. That was much better but the old 'sideways shuffle' was lost and units moving sideways turned their flank to the enemy That was not so good. The only thing which I miss from MTW is the ability to drag out units and r/click to achieve equal depth.
I would suggest experimenting with a few custom battles and I an sure you will find that things are just as easy, if not easier, to control

......Orda