Interesting. We do have some code that should install fonts to the target directory, when Qt has installed them to the staging directory. From qt5base.mk:

define QT5BASE_INSTALL_TARGET_FONTS
        if [ -d $(STAGING_DIR)/usr/lib/fonts/ ] ; then \
                mkdir -p $(TARGET_DIR)/usr/lib/fonts ; \
                cp -dpfr $(STAGING_DIR)/usr/lib/fonts/* $(TARGET_DIR)/usr/lib/fonts ; \
        fi
endef

So maybe the specific configuration you're using does not install fonts to the staging directory.

Can you post your Buildroot .config file?

Seems -fontconfig disables font installation:

$cat qtbase/mkspecs/features/qpa/genericunixfontdatabase.prf
CONFIG += qpa/basicunixfontdatabase
contains(QT_CONFIG, fontconfig) {
    DEFINES += Q_FONTCONFIGDATABASE
    LIBS += -lfontconfig
} else:!android {
    fonts.path = $$[QT_INSTALL_LIBS]/fonts
    fonts.files = $$QT_SOURCE_TREE/lib/fonts/*
    INSTALLS += fonts
}

I also don't build freetype now, but have not tested if it's a requirement.

Overall, it's more a question of documentation.

Indeed with BR2_PACKAGE_QT5BASE_FONTCONFIG selected no fonts are installed in staging and essentially they won't get on target.

Moreover even if one puts fonts in /usr/lib/fonts on target they won't appear in Qt5 apps.

So I had to rebuild entire Qt5 after I disabled BR2_PACKAGE_QT5BASE_FONTCONFIG and fonts appeared in Qt apps.