epub (Sigil) to mobi: TOC nodes missing / not functioning

[TheBFG]

I've searched around in the forums, but haven't found anything matching this problem. Here 'tis.

Ordinarily I mark up section headings with e.g. <h3> or <h4> tags in Sigil, then let it populate the toc.ncx file to create a table of contents. I do NOT usually have Sigil create an additional html TOC. I save as epub, then use Calibre to convert to mobi. The TOC is ordinarily preserved.

Today it wasn't. To be more precise, when I proceeded as usual and then open the mobi file in Calibre's E-book Viewer and click the TOC icon, a TOC tree appears but one that is missing the leaf nodes. Similarly, when I scroll to the end of the e-book, the html TOC there is missing the entries for the same nodes. The links that are there do work, as does clicking on the ones in the TOC pane.

If I now go back to Sigil and have it generate an html TOC (which I notice includes all the headings I marked up), save to epub, and use Calibre to convert the new epub file to mobi, I get a different result. The TOC pane in the Viewer now shows all the nodes in the TOC, but none of them is functional. Similarly, none of the links in the TOC at the end of the ebook function either.

Curious.

"Force use of auto-generated TOC" is unchecked. The XPath expression boxes are empty.