Hello, I'm super sorry to hear that! If your device is running low on storage there is unfortunately a high chance of your browser deleting the games data, I know it is frustrating and I've attempted to fix this many many times, but since this is a browser related issue there is unfortunately not much I can do.
The only solution at the moment is to regularly backup your save using Settings > Save Management > Export
If you have any screenshots or your old UID I might be able to recreate a stripped down version of your previous save file for you, so please let me know!