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Unfortunately I don't know anything about Sigil (or making books in Calibre) but there are forums for both on this site (further down).
My guess, just going by my very limited book compiling experience with AsciiDoctor, is that the "binding" document that holds links to all the chapters, doesn't have Table of Contents attribute defined. That's one of the things I had to do in AsciiDoctor when compiling my sample book.
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