GtkScaleButton has the wrong parent_class
Currently, GtkScaleButton doesn't subclass GtkButton anymore but the GtkScaleButtonClass still defines the parent_class to be a GtkButtonClass, see https://gitlab.gnome.org/GNOME/gtk/-/blob/master/gtk/gtkscalebutton.h#L62, here's the resulted gir file
<record name="ScaleButtonClass" c:type="GtkScaleButtonClass" glib:is-gtype-struct-for="ScaleButton">
<field name="parent_class">
<type name="ButtonClass" c:type="GtkButtonClass"/>
</field>
</record>