I don't use any fonts explicitly. It's a problem with the ruleset. I'm probably using a control template inherited from PF2 or CoreRPG that is still using FGF fonts.
EDIT: The control I'm using (button_text_large) uses this font:
Code:
<font name="button-white-large">
<fgffile name="graphics/fonts/regular-11.fgf" />
<ttf file="graphics/fonts/Noto_Sans/NotoSans-Regular.ttf" name="Noto Sans" size="14" />
<color value="#FFFFFF" />
<shadow offset="1,1" color="#000000" />
</font>
Which seems to specify a TTF variant. Any idea why this wouldn't work in FGU? There isn't a ton of documentation on fonts in the wiki, none of the examples there have both <fgffile> and <ttf> elements so I'm not really sure how FG handles that. It looks like most of the fonts in CoreRPG are defined with both elements so I don't think that's the issue.