Same thing happened to me today, although on PS5. Was on chapter 11, saved manually as always, but the next time i booted it up I had only a save game on chapter 8. I have no idea how it happened and this post is the only one that reports this issue. Really anoying cause ill probably not bother redoing any secondary stuff (which i had done a lot between the two saves) and just speed through to the end just to get this thing done.