Andy is possible that you had the draft open in two different windows or tabs?
I have found (via some testing) that the most-recently closed window is what is saved. I've never lost a draft but have screwed up and had one open in different windows and exited my older version last (so it replaced my just--minutes-earlier-edited/completed version.) For it to save that way IT APPEARS that the second window actually needs to be edited.
So, open a draft in window A (draft A)
Open the same draft in window B (draft B)
Edit draft A and close window A = draft A is saved.
Then exit window B = draft A remains (draft B is not saved)
BUT make ANY edit to draft B before closing window B = draft B replaces draft A (A bummer if that happens by accident)