This is almost certainly caused by something outside of our power. We do not get to decide what "cleans up" browser data. If you run any "cleaner" or "optimizer" programs they will clear your cache. If you are on mobile your phone can clear it in the background when space is low. There is nothing in the game code that clears all your saves. The only code that deletes saves at all is the [Delete] button in the save menu. This is why you should always save to file between sessions.