Opening a DOCX with footnotes which use Character instead of automatic numbering, the footnote text gets prepended with a copy of the character.

1. In a new Writer document, insert a footnote using Insert->Footnote and Endnote->Footnote or Endnote...
2. In the dialog, choose (*) Character, and put e.g. "a" there. Click OK.
3. Save the document as DOCX.
4. Test with some different application, e.g. MS Word, that the document is correct, and the footnote has only the "a" mark and no text.
5. Open the document in Writer

=> the footnote has "a" mark, and "a" character as the footnote's text.

Saving the document, the character gets written there. Repeated open-save-close will multiply the copies of the character.