Using Gtk.FontButton in Glade causes deprecation error, but can't eliminate it

Question

This is a more precise specification than the one I asked in a previous question.

In this case, I generated a dialog, which has several Gtk.FontButton definitions. The target Gtk version is 3.24. On saving the glade UI file, I get the following:

[conf_window > GtkBox > GtkNotebook > conf_window_font_page > GtkFrame > GtkBox > GtkBox > conf_total_fontbtn]
    Property 'Font name' of object class 'Font Button' is deprecated

In Glade, a warning triangle appears left of the entry in the widget tree. In the object property window no warnings are shown in General, Packing. In 'Common', there are triangles at 'Double buffered' (set to True), and 'Left' and 'Right' (both set to 0). This should be right.

The corresponding XML generated in the UI file:

    <child>
        <object class="GtkFontButton" id="conf_total_fontbtn">
            <property name="visible">True</property>
            <property name="can-focus">True</property>
            <property name="receives-default">True</property>
            <property name="font">Sans 12</property>
            <property name="language">en-us</property>
            <property name="title" translatable="yes">Seleccionar fuente</property>
        </object>
        <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
        </packing>
    </child>

The only property I suspect related is <property name="font">Sans 12</property>, but clearing the field in Glade results in changing it to <property name="font"/> - it doesn't eliminate the property (if that is the problem).

According to the class reference, font is the right property.

• Why am I getting this error message?
• And can I somehow eliminate it?
• Or is this a Glade problem (I am using Glade 3.40.0)?