Item description:

On an overworld map, battles are determined by choosing a formation from a group within 32×32 tile zone. The game calculates the zone based on the coordinates of the party.

However, the zone calculation is not run on each step, but rather on each redraw of the map.

Examples of redraws include exiting battle, exiting the menu, and exiting a town.

It is easy to see this bug by entering and exiting the menu at a boundary where the enemy formations change, then walking into the adjacent zone until a battle is triggered. Monsters will appear from the previous zone.