Uploaded experimental editor V1.26-D. This includes the full implementation of autosave (ie: you can now configure the timing or turn it off altogether, and old autosaves can be auto-deleted). It also finally includes alias support, meaning one less situation in which Cleanse Levels is necessary to get a level to open in the editor. That aside, it fixes a couple of other minor bugs. Attached to first post.
@mantha16: This might fix your background bug too. Not sure. I did find a case where backgrounds weren't displaying correctly (though they would still ultimately be saved correctly in the level file, it was purely a visual bug); it's very unrelated to the circumstances you've described but still could possibly make a difference.