Sorry for your loss of effort. I have seen these types of things happen when, for example, one loses connection with the network while submitting. And the cache settings on these pages mean that when you go back, you have lost your data.
One thing that might be happening is that the session on your server was timing out (perhaps after two hours) so the server lost it's association with you.
In any case, you might want to file a bug report in Moodle Tracker on this issue. I suspect that better settings for the cache instructions may at least allow one to go back and save work, and making sure that sessions don't timeout or can get reestablished might also be important.
In any case, I always tell my students to select all and copy before submitting anything long into a web form -- Moodle or otherwise.