I did a multiplayer test this evening and intentionally sent corps far forward to defensive lines. I had no problems whatsoever. A couple of times they had to do a bit of fighting, but I trusted my corps commanders and after they'd pushed the enemy back a bit, they formed defensive lines properly.
You'd have to be drawing your defensive line within 200 meters of the enemy (or at least where he'll be by the time you get there) to have any kind of a problem with this.
It would be possible to mess this up if you sent your forces against a much larger force and your guys never got to form their lines. You'd almost have to do it on purpose though.
There are places on some maps where the corps commander will defend farther forward than you want him to. Your mission, should you decide to accept it, is to figure out what kind of terrain causes this behavior and avoid forming defensive lines too close to the enemy if it's in that kind of terrain. Good luck.
Hook