Kindly describe the area where you had a battle that resulted in inaccessible placement of armies. The most precise description would be to hover your mouse over the spot you attacked or were attacked (unless it's a settlement, then simply state the name). Then pull down down the console by pressing the tilde key ( ~ top left) and then typing show_cursorstat. Note the co-ordinates and then describe the direction of attack together with who got stuck.

A simple 'smoothing' of the area in map_heights should then rectify that problem.