See the implementation of gtk_entry_create_layout():
pango_attr_list_splice() is used to add the PangoAttrList of the preedit
string. And that is done *after* applying the PangoAttrList of the
"attributes" property.
https://bugzilla.gnome.org/show_bug.cgi?id=776868
*
* This is mainly useful to change the size or weight of the text.
*
+ * The #PangoAttribute's @start_index and @end_index must refer to the
+ * #GtkEntryBuffer text, i.e. without the preedit string.
+ *
* Since: 3.6
*/
entry_props[PROP_ATTRIBUTES] =