Created attachment 169090 [details]
Unmodified file created by LibreOffice

This bug is also present in 6.4.7.

Steps to reproduce:
1. Create a new file with a footnote.
2. Add a cross-reference to the footnote: first its page number, then the footnote's number.
3. Save to DOCX.
4. Close and reopen.
Result: The reference to the footnote's number disappeared.

I investigated a little the DOCX archive. Here is the culprit in word/document.xml:

      <w:bookmarkStart w:id="0" w:name="_RefF0"/>
      <w:bookmarkStart w:id="1" w:name="_RefF0"/>
      <w:bookmarkEnd w:id="1"/>
      <w:r>
        <w:rPr>
          <w:rStyle w:val="Ancredenotedebasdepage"/>
        </w:rPr>
        <w:footnoteReference w:id="2"/>
      </w:r>

There is no <w:bookmarkEnd w:id="0"/> in any file in the archive and both bookmarks have the same name. I suppose that we should have something like this instead:

      <w:bookmarkStart w:id="1" w:name="fnRef"/>
      <w:r>
        <w:rPr>
          <w:rStyle w:val="Ancredenotedebasdepage"/>
        </w:rPr>
        <w:footnoteReference w:id="2"/>
      </w:r>
      <w:bookmarkEnd w:id="1"/>

where the bookmark elements with w:id="0" would be placed in word/footnotes.xml (at least it is what I observed in a document produced by Word 2010).

Additionally, the footnote reference is called by:

      <w:instrText> REF _RefF0 \h </w:instrText>

We should have (provided that we changed the w:name attribute):

      <w:instrText> NOTEREF fnRef \h </w:instrText>

These are only hints, and I may be mistaken at some points. I produced a modified version of the file with the fixes mentioned above. When I open it in Word 2010, it complains that it is corrupt, but if I force opening, it basically works. References are here and can be updated correctly if I modify the file.