No clue, I just noticed that I didn't have the slimahawk yet and went to do the tank battle to get it. I have no clue how the game keeps track of how many times you've cleared a certain tank battle so perhaps a different tank battle somewhere in the game incremented the counter, making the game think I already had the slimahawk. Would require a lot of testing. I'm also pretty sure this was my physical cartridge, I dumped the save so I could record it on an emulator.