[DNSB]

For what it may be worth, your ePub did not contain any section files with names such as whatever_split_0001, see the attached image. I took a couple of seconds to open the file in Sigil and use it's ToC tool to regenerate the ePub3 nav document and the ncx ePub2 nav document for the h1/h2 header items. It seems to move to the section headers now in Sigil and calibre's viewer.