Hello, this is because you are playing a webpage (4399 mini-game version). Since it is not a stand-alone download version, your computer's cache file will be cleared by default after the first save. In layman's terms, your archive was deleted by 4399, so they have no obligation to save the game for you, so you can only download it to play.

Besides, Plants vs Zombies does not have the second version, only the Great Wall, the ordinary version and the annual version.