GTK4 doesn't show mnemonics underline in a button - c

Question

I want to set a mnemonic underline in a button called Quit, but this underline is not displayed though keyboard acceleration works by pressing ALT and the specified mnemonic key. In the code I have set use-underline to true. but it just doesn't work. I want the Q character to be underlined so that the Q character is recognizable as menemonic to the user. What am I doing wrong?
Here is my simplified main code:
#include <gtk/gtk.h>
#include <glib/gstdio.h>
static void activate (GtkApplication *app, gpointer user_data) {
GtkBuilder *builder = gtk_builder_new ();
gtk_builder_add_from_file (builder, "builder1.ui", NULL);
GObject *window = gtk_builder_get_object (builder, "window");
gtk_window_set_application (GTK_WINDOW (window), app);
gtk_widget_show (GTK_WIDGET (window));
}
int main (int argc, char *argv[]) {
#ifdef GTK_SRCDIR
g_chdir (GTK_SRCDIR);
#endif
GtkApplication *app = gtk_application_new ("org.gtk.example",
G_APPLICATION_FLAGS_NONE);
g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
int status = g_application_run (G_APPLICATION (app), argc, argv);
g_object_unref (app);
return status;
}
Here is my builder code:
<?xml version="1.0" encoding="UTF-8"?>
<interface>
<object id="window" class="GtkWindow">
<property name="title">Grid</property>
<child>
<object id="grid" class="GtkGrid">
<child>
<object id="quit" class="GtkButton">
<property name="use-underline">true</property>
<property name="label">_Quit</property>
<property name="has-frame">true</property>
<layout>
<property name="column">1</property>
<property name="row">2</property>
<property name="column-span">1</property>
</layout>
</object>
</child>
</object>
</child>
</object>
</interface>