LocalStorage Data is erased when closing an app, why?

I doubt the data is erased. Most likely it's get overwritten when you re-open the app. You need to post your project file or a screenshot of your events where you read and write to Local storage.

I'll get the screenshot soon, but the main idea I'm thinking of (it's likely not the cause) is that the data is loaded on start of layout, and maybe that doesn't trigger when opening the game, causing the data to then not load and then being saved at 0

Probably wrong, but I have no idea what else the problem would be.

Does this bug happen when you preview the game in the editor? When you restart the preview, is the data from LS loaded correctly?

That's the weird part. If I use Preview on laptop or mobile, it works perfectly fine, but with the app, it never works.

If you can provide the cp3 file then I can take a look at it.

drive.google.com/file/d/17ZzSpfkN9ksOq6zXU-r-2EIvPGC1EoFZ/view

I hope this works, I haven't shared a c3p file ever, so if this doesn't work, let me know.

Your mistake is that you are saving values in LS every 0.1s

I guess it works on PC because it's fast and it takes less than 0.1s to load the data. But on mobile the LS gets overwritten with default values before the old data is loaded.

You should only save values in LS when needed - once at the end of the level.

Okay, I'll see if it works.

Would it be better to use "On End of Layout" instead of "Every 0.1 Sec."?

Yes, you can use "On end of layout"