So checking out some of the ePubs for my stories on my dad's Kindle reveals a lack of chapter headings in the TOC on that device.
I use Sigil to construct the ePubs and build out a TOC (which shows up on my iPhone's Books app), but understand there are differences in how Kindle does things.
Is there a regex (or whatever) string for Calibre that I should use when I "convert" an ePub to a new ePub? I do make the AZW3 and PDF conversions but can't test those with Kindle or the Kindle app.