Narrative Reporting Book TOC Suddenly Displaying Blank After Years of Normal Function

All of the client's Narrative Reporting books have suddenly started displaying a blank Table of Contents. When I recreate a brand-new book using the same TOC.docx file, the TOC generates correctly. However, on the existing affected books, the issue persists even after saving the book under a new name or deleting and re-adding the TOC artifact — the TOC section only shows the header "Table of Contents" with no entries below it.

Recreating a single book from scratch would be a workable fix, but this is affecting every existing book — there are dozens of them — so that isn't a practical solution at this scale.