I'm finding it's best to detach as few units as you can get away with, if there are too many away from the Corps Command it becomes a nightmare keeping track of them....one or two key units to mirco manage is enough for me.
I notice several units get detached at the start, something to do with the deployment area's I'm guessing.
Each Unit you detach, has a heavy impact on the strentgh and the units the Corps Commander has available to deal with his orders - which means he has to adjust his line accordingly - which might result in complete and utter chaos, because he is simply lacking the proper units to fulfill his task...
If I detach a unit...then only to occupy a village or to scout...
but this is only my approach....
As to the detached units in the deployment phase...I saw it happens if you move your Corps over a village...it usually leaves a unit in there to occupy it and then this unit get's detached...I haven't checked it thoroughly though...