Bold or Italics Formatting Doesn't Show if a Font is Applied

I’m writing a Vala/Gtk3 app that incorporates a TextView widget allowing the user to format text with bold, italic, color, font, etc. using toggle buttons… Everything works just fine until I start applying font formatting to the text. If I format a sentence as a given font, that works but if I then apply bold or italic to a word or two withing that sentence, this formatting doesn’t appear. I can include the code here but ultimately, it seems the problem has got to be some fundamental misunderstanding on my part as the generated serialized XML looks fine (at least to me, excepting the one “empty” tag):

  <tag id="1" priority="0">
  <tag id="0" priority="5">
   <attr name="family" type="gchararray" value="Serif" />
   <attr name="size" type="gint" value="12288" />
  <tag name="bold" priority="1">
   <attr name="weight" type="gint" value="700" />
<text><apply_tag id="0"><apply_tag id="1">Just <apply_tag name="bold">some</apply_tag> text.</apply_tag></apply_tag></text>

Does anyone know why the bolded word doesn’t appear? Incidentally, the exact same thing results if the one word is bolded first and then all of the text is set to the font. Still, the bolded word doesn’t display… Thanks!