gtk_window_fullscreen_on_monitor does not work

The standard gtk4 utility fullscreen_on_monitor, on a 3 monitor setup, to have a window open on monitor 1 and monitor 2:

display = gdk_display_get_default();
monitors = gdk_display_get_monitors(display);
GdkMonitor * monitor = (GdkMonitor *) g_list_model_get_item(monitors, 1);
GdkMonitor * monitor2 = (GdkMonitor *) g_list_model_get_item(monitors, 2); 
gtk_window_fullscreen_on_monitor(GTK_WINDOW(lwindow), monitor2);
gtk_window_fullscreen_on_monitor(GTK_WINDOW(rwindow), monitor);

launches both GTK windows onto the same screen, regardless of which monitor is selected.

The function is documented here: https://docs.gtk.org/gtk4/method.Window.fullscreen_on_monitor.html