[KevinH]

Edit: Skip this response, see my later responses below:


Open the epub in Sigil or Calibre's editor. Then navigate to the actual nav xhtml file. Does it exhibit the same issue? If not, then perhaps Books is finding an older incorrect toc.ncx (NCX) and displaying that instead of the NAV. In addition, the book may have an html toc xhtml file that was meant to be read by the user just like book content that is separate from the NAV and/or NCX that might be the issue. If so try loading that file in Sigil or Calibre's editor to verify it is correct.

If all possible sources for the info shown in the reader's TOC window are correct, then you know the bug is in the reader's code that parses the NAV or NCX.