I add a line and some text, save the document, do some more changes,
but closes the document and chooses “Dont save”. Everything before last manual save is saved nonetheless, and is retained upon re-opening the document:
So what you are saying is you expect, open a Saved file, work on it saving regularly, then close it and choose not to save it at that point. Now when you open that file again it will be as if you have made no changes to it.
If that was the case, what is the point of saving anything while working.
However, if you open a saved file, make some changes, choose save as and save that, continue working and eventually close the file the original saved file will remain unchanged but the save as file will have the changes.