I'm having similar issues, I think that the "C:\Users\[Username]\Saved Games\Grounded Windows 10" is no longer used by the Windows version of the game since the December 2020 update.
This is a problem when trying to import saves to the Windows Version.
The Steam version still uses "C:\Users\[Username]\Saved Games\Grounded" which is a bit odd.
I think that those files are juste temporary game state and not actual saves.
One option you could try would be to save on game on the steam version.
Then Copy the content of the save you want to the folder of the steam version and replace the IDs if they're different.
The Steam version saves are at C:\Users\[Username]\Saved Games\Grounded