Quoting François (main dev) from another comment: "The game was designed around the nes mapper 30 and from what I understand (I could be wrong) does not support traditionnal battery save, only self flashing save. Which means that if the emulator does not support flash save, in game saving will never work on this emulator until they release an update that support self flashing save. Sorry. We will update the itch page to make this more clear."