I suggest to default to adding that to any peacy treaty offered by the AI. I guess many will overlook that and cause grumpy reloads.
I also had an army in one province I accidentally gave back. This army got into an invisible battle, lost it and was expunged into my territory, severly hurt. IMHO in the case of a peace treaty that shouldn't happen. Expunge yes, fatigued yes, but no losses. This was an agreed peace.
It would also be nice if the player could choose where to retreat to. Worked in my case but I can think of a lot of situations where auto-retreat goes wrong.