That is actually a restriction of the QR rendering kit that is used by Sigil and Calibre. There is a font bug in there. It has been the case for quite some time.
That being said, I have two additional comments. You embed fonts I see. If you don't have permissions from the license owners of the fonts, it would better to remove them. In general it is better not to use fonts for body and standard paragraphs and use the standards of the readers. Better remove them.
The other is about the smallcaps. The CSS option 'font-variant: small-caps' is ignored in most readers and will not result in smallcaps. Most ePUB makers resort to creating fake smallcaps with a reduced fontsize.