Whether you do a manual Save or not, if you Quit krita normally then the autosave file is deleted by krita.
The autosave file isn’t made for you, it’s made for krita to recover from a crash. If you Quit normally, krita assumes that you’ve saved your work so it deletes the autosave file and finishes.
If you were offered an autosave file when you started krita then krita must have crashed in the previous session.
Another possibility is that krita was ‘killed’ by the operating system for some reason and didn’t have chance to delete its autosave file.
Also, if you make changes to an image and try to Close it or Quit without saving, you’ll be given a warning notice and asked if you want to save it.
Do you remember that happening?