Game doesn't save progress on exit with the windows version for some reason.
Viewing post in Plant Girls: Insect Invasion comments
It looks like the save gets replaced by a Normal difficulty game with no progress after doing the following: Close Game > Open Game > Settings > Adjust any volume slider > Apply > Continue (your save will be gone).
So avoid changing your sound using the in-game settings so you can keep your save progress and instead adjust the application sound using the volume mixer or your device sound on mobile.
Hope this helps!