Sounds like a crash. Not sure what's causing that, probably a bug somewhere in the base code. I really need to write a crash handler to gather and upload memory dumps, but until that's a thing, it would be helpful if it's possible to figure out a way to deliberately trigger this.
If you're crashing a lot, one thing you can do to minimize the amount of work lost is to periodically exit out of edit mode then go right back in. The base is committed to the database (and saved to a backup file) whenever you exit the editor.