A family of Microsoft word processing software products for creating web, email, and print documents.
If the appendix headings have been inserted as captions, then you should use References | Captions | Insert Caption and select the Appendix caption label. This will insert a SEQ field with the Appendix label, which is undoubtedly what the List of Appendices (a Table of Figures) is based on:
If you press Alt+F9, you will see the caption label: Appendix { SEQ Appendix \ ARABIC }*
and the TOC field code: { TOC \h \z \c "Appendix" }