From the little looking around that I've done myself I've found out that these games don't rely on a battery to store the save files, so that can't be the problem. I've bought the game secondhand with my Gameboy advance about 5-7ish years ago, the game is in German (not my native language) so at the time I was younger and never really got out of the second town. Later I finished the game on an emulator(in English) so I can't recall if it had the problem when I first got it since I didn't speak the language and never knew what button needed pressing to save the game.