i tried a no zombie sandbox to test some stuff, modified the zombie count and pressed save. after that i went into apocalypse and there were no zombies.
i went back into sandbox and noticed that sabing actually overwrote the main apocalypse setting instead of prompting a new slot. trying to reverted the change ended up in the game asking me to save a new preset.
deleting the zomboid lua folder fixed it, but i can easily see people falling for it in the future