The calibre VIEWER has its own settings. Tap Esc. to see those.
The font section seems to only apply to unstyled fonts, so than might not fix your issue.
You could try editing the books EPUB Stylesheet Using the editor (T)
2 general places might help: change the
font-family: used for the Bolded style
Code:
font-family: Alegreya, serif;
to a different font or just leave the generic (serif)
or change the font-weight for the style (assumes a
span,
b or
p controls)