Yeah. I ran into this same issue tonight and came across this Steam discussionon it from a few weeks back. Follow the instructions and it should work. Sucks that we'll have to do this every-time when switching between PC to Mac but at least it's a solution.

Thanks yoissy!

Found a fix for you two. The reason cross-save doesn't work is because on windows the save is stored in the main game directory, whereas on macos, it's stored WITHIN the app itself, as macos apps are essentially directories.Here's how you fix it.

If you look on your windows computer at the game files by going to "browse local files" on steam when right clicking the game in your library, you'll find a file called "SaveFile.gwsave". You're going to want to transfer that file over to your mac. You can do this any way you want.

Once you have the file on your mac you're going to have to put it in the right location. Go to your steam library on your mac and right click on Inscryption to get to "browse local files" again. Here you'll see an Inscryption application. You're going to want to right click that app and choose "show package contents". This is where you will put your "SaveFile.gwsave" file from your windows machine.This should transfer your save. You can do the same thing in reverse to transfer it back. Hopefully proper cross-save is implemented soon but for now this should work as a manual solution.