[KevinH]

Yes a longstanding but luckily basically harmless bug. When you generate a toc and manipulate things by selecting and omitting headers, Sigil add a marker class "not included" in toc header attributes to record the fact this header is not included or if you promote or demote a header, or change the title of the header, Sigil must parse the file to make those changes and then saves it.

Sigil neglected to run it through the CharToEntity conversion before saving that file. So some unicode chars were not properly converted back to their entity equivalent.

Just running Mend All will put back your Preserved Entities, so nothing was ever lost just converted to its actual unicode character equivalent.

That bug fix has now been pushed to Sigil master repo. That fix will appear in our next release.

Thank you for your detective work on this bug, which allowed me to recreate it and greatly narrow down where it might be.