That's odd, both the mac version and windows version are derived from the same project file.
Are you starting from a new save game each time? Something's get changed between versions that breaks old save files in weird and wonderful ways.
Please let me know if you're using a save file from a previous version.
Also you say you can't properly download from the Itch.Io app, but can from mega? Are you actually receiving and error message when extracting the zip file, or just guessing that, that's the cause?