A family of Microsoft word processing software products for creating web, email, and print documents.
This has come up before and it's likely to be a bug. MVP Doug Robbins has suggested a workaround: Set the Line Spacing for the endnote separator to an exact amount (and make sure that amount is smaller than the font size).
To display the endnote separator, do the following: Switch to Draft view. On the References tab, click Show Notes. In the endnote pane, click the drop down to display the endnote separator.