I'm not here to defend the way the AI deploys units to front lines, I actually think it's rubbish and one of the worst aspects of a shaky AI, just to give advice to a fellow player; don't ever use a front line when you're defending. If you're playing at speed 1 you've got enough time to just assign every unit manually even if you're on the Eastern Front and your defence will be far stronger if you do. Find a decent line to hold (good terrain bonuses, straight, few adjacent provinces), fall back to it, dig in, wait for the enemy to grind itself down, then assign a battle plan and build up your bonus when they look weak. If your units are in combat they won't build up their planning bonus anyway so there's literally no benefit to having a front line when you're on the defensive.
PS. Going fully defensive in multiplayer probably isn't a good idea but seeing as you're playing on speed 1 I'm assuming you're playing single player.