Strange.
I can see my selection being entered in config file "tweak_book_gui.json" for option "preview_standard_font_family":
- "preview_standard_font_family": null, for the default serif font family;
- "preview_standard_font_family": "mono", for the mono font family selection;
- "preview_standard_font_family": "sans", for the sans-serif family selection;
But despite this, the font family shown in the preview window remains "serif", which is also the default.
I can change the "font family for serif text" setting, though, which is immediately shown after pressing "ok", but that is it.