Hello. I'm a newb so forgive me if my dd isn't up to snuff. I have a custom content type which I make an entity reference (unlimited values) to from my "Basic Page" content type. When setting the sort by to "A field attached to this entity", I was picking a selection that, in retrospect, was impossible. When I then went to create a new page, the entity reference field had no options available to select from. It took me awhile to realize I was sorting by an impossible field, and as soon as I corrected that problem I had the correct entities available for selection again.

What this illustrates is that it would be best to limit the "sort by" selections to those selections that are actually possible to use.