After setting the font in "Configure Kate", and selecting a "normal" style of any font, bold and italic fonts no longer show up. This appears to be an issue with how configuration files are set. See here: https://forum.kde.org/viewtopic.php?f=17&t=139868 and here: https://forum.qt.io/topic/78036/bold-fonts-not-working-for-many-fixed-width-fonts-in-linux/ The second link shows how the $HOME/.config/kdeglobals config can cause this bug on all qt5 applications running in Plasma 5.9
See bug 378523.