Gtk-doc to PDF shows UTF entity for certain glyphs

Hi,

I do for libmonothek:

./configure --enable-gtk-doc --enable-gtk-doc-pdf

I get following output in my final PDF.

GObject

╰── MonothekQrcode

Howto fix missing glyphs?

Here is the PDF .

The glyph shows in gucharmap

gucharmap

regards,
Joël