Your astute question made me think a bit and allowed me to nail down all the issues concerning these features. I play almost exclusively in historic mode. Whereas I assume you mostly play with custom settings.I have my courier level set to 'brigade', what is yours set to?
The one difference between the two is that couriers are not sent to regiments from an AI commander in historic mode. Never. That's why I see the regiments moving immediately. This is an oversight and would be a good idea to change, as historic mode should reflect historic fighting.
Now the interesting bug. Player is corps commander. My typical order is: move to a location, face your unit in a direction, change to a certain formation, assume a stance, use roads. All as one courier order. Try this in either historic or custom mode, it won't matter. You will see your brigades and regiments begin moving as soon as the division commander gets his order. Now write the order without the"face your unit in a direction." Now the sub-units will wait for their courier to arrive before moving. The "face your unit in a direction" is buggy.
That should be enough info for Norb to find the problem.