I have the same problem, restarted twice and hard saved but as soon as I start the game again all save files are lost. hope someone can help.

apparently it's in the documents, my games, and you will see kingdom hearts 3 go into it go to steam until you see the 000000 file and click in there and look for your saves then copy them and paste then in the 76561199127740944 file and it will load your saves. I tested it and it works it will load the saves from that point on it seems.