[KevinH]

My guess is your e-reading device reads only from ncx to get the toc, many old epub2 only ereaders do that, but under epub3, the info you changed was written to the nav file. You can run an epub3 only Sigil tool to manually sync the official nav with the now deprecated and legacy ncx.

I personally would think about moving to an e-reader that actually supports both epub3 and epub2, and not just epub2. It has been almost 15 years since the epub3 spec was released.