JabRef version 3.7 on Windows 10

JabRef's sort order is influenced by whether the reference entry contains the "crossref" field. In the output, all of the references that do contain this field appear first (sorted within this group). Then, all of the references that do not contain this field follow separately (again, sorted correctly within this group).

Steps to reproduce:

1. Load a bib file that includes some references of type @InCollection for which a "crossref" field is specified.
2. Under the File menu, "Database properties" window, use "Save sort order" to request sorting by bibtexkey.
3. Save the file. Regardless of their bibtexkey's, all of the references with "crossref" fields will be at the top.