@jbennion. Please, go to the dedicated thread in
Calibre > Conversion (moved from Editor by BetterRed) and provide info about your Calibre settings while doing the original MOBI to EPUB conversion.
I mean, yes, we are completely sure Kobo ACCESS has a rendering bug (
another one more) about handling those missing fonts. But your case might also show a Calibre conversion issue. Please
provide all the info you can about HOW you've done the conversion (which settings you have used), and
specially check (confirm or deny) what jackie_w is suggesting which might be a good guess given the device info you have posted in your profile (that you own a Sony PRS-650).
OTOH,
be careful while hunting book bugs with Sigil. Sigil rebuilds the whole book, (and it completely modifies its internal structure), just when you open it, so Sigil *
might* hide the original issue. Calibre Editor is much better suited for this task because it does not modify the book in any way unless you explicitly perform actions with it, (well, at least, it doesn't perform any modification in the book if you deactivate "metadata sync" and "auto-beautify files on opening").