Usually that happens when on EPUB Export Options/Navigation, "File Name" is selected. Change it to TOC Style.
Yes, sadly, I already checked that. I thought would be the issue, but I had 'Multi Level (TOC style)' selected each time.
If the TOC style is not set up in the style source document, file name toc is generated instead. Did you set up TOC Style in Style Source Document of the book?
Also, the same style has to be selected in the Navigation drop down during Reflowable EPUB export.
This was my problem! Thanks so much for your insight!