Maybe an error or intentional? When more party members but not in combat roll, MP is used by party members not seemingly active in the combat, the ones that can be swapped out for other members. Especially interesting when going through that dungeon or cemetery with poison spikes, all the non active members also were brought down to one hp. And for players sake, label the weigh points on maps, even the ones not visited yet, having names will help with navigation. Some places have more than one name and its confusing as is. The mini map could give the area name too, navigation is poor without simple aids.