That has also happened to me when continuing a draft that I started at a different time but left the wordpress page in the browser (Firefox) open. It's not the first time. The autosave does not seem to work when this happens, and when I click Save Draft or Preview, it gives that message and wipes out everything I wrote (even if I don't respond to the message and go Back) and reverts to what was there to begin with. This has happened three times on the same post, and I've had to start all over again. I'm sure I was still signed in, and this last time I made sure to click Save Draft several times as I was writing, but I didn't see that it actually did not save it until it was too late (and of course I don't remember to copy what I wrote before I click the preview or Save Draft).
It takes me a long time to write what I write, and It's very frustrating because I have to go back to my notes and photos and try to remember what I wrote.