Furthermore / to elaborate - it seems you need to save in a new slot each time to reroll. For my method I am saving at the end of the map before proceeding, completing everything I need to complete in-chapter and emptying my active party, saving, proceeding to next chapter, checking the menu, loading the previous save and then saving into the next slot... repeating this process until I've filled every slot except the map end save, loading that save and starting over.