Hi, it seems that saves made or modified with Proton no longer work when transported back to Windows. The game posts the "Failed to load save" error when attempting to load a proton save. The saves aren't corrupted, but only work on Linux running the same version of Proton they were created with.
I believe this is only true of saves made/modified with newer versions of proton (Version 6+? I can get a more specific answer if necessary).