[meme]

This was reported as a Sigil issue, and while it will be fixed there, it seems that Calibre has the same behaviour when using a TOC entry that points to just a filename instead of an id.

Open the attached in Calibre's E-book Viewer, then open the Table of Contents. Scroll to the bottom of the file. Click on Chapter 1 - it works ok. Click on Chapter 2 - it works ok. Scroll to the bottom of the file again. Click on Chapter 2 - the text shown doesn't change. The issue appears to be that the TOC src attribute only points to a filename not a filename#id. The link works ok if you are displaying a different HTML file, but if you are on the same page as the heading then the page is not updated.

The TOC entry appears to work ok on a Kobo and when converted for a Kindle, so it seems valid if not too troublesome. As for why there is no id; Sigil links to the name instead of the id if the heading is "close" to the top mainly to make sure anything above the first heading, like an image, in a chapter is always shown.