Try to avoid using Fast Travel as much as you can early in the game and instead grind your way to the top as there is no alternative way to complete some of the mid-game missions.
To exploit this bug, just access the Active Members sub-menu of the Party menu, and select "Confirm Changes". Your party's full HP will be immediately restored. This is useful for exploring during damaging weather conditions, or if you want to very slowly cheat your way through lava.