Comment 6 for bug 1492443

I've found the problem. The function display_pincode_callback in panels/bluetooth/gnome-bluetooth/wizard/main.c is empty so the gtk ui wont show the pincode (like for the passkey auth).
Quick solution is to change the function to:
static void
display_pincode_callback (GDBusMethodInvocation *invocation,
     GDBusProxy *device,
     const char *pincode,
     gpointer user_data)
{
 gchar *text, *label;

 display_called = TRUE;
 gtk_assistant_set_current_page (window_assistant, PAGE_SSP_SETUP);

 replace_target_properties_for_device (device);

 gtk_widget_show (label_ssp_pin);

 if (target_ui_behaviour == PAIRING_UI_KEYBOARD) {
  label = g_strdup_printf (_("Please enter the following PIN on '%s' and press “Enter” on the keyboard:"), target_name);
  text = g_strdup_printf("%s⏎", pincode);
 }
 else {
  label = g_strdup_printf (_("Please enter the following PIN on '%s':"), target_name);
  text = g_strdup_printf("%s", pincode);
 }

 gtk_label_set_markup(GTK_LABEL(label_ssp_pin_help), label);
 set_large_label (GTK_LABEL (label_ssp_pin), text);
 g_free(text);

 g_free(label);

 /* Reject all the calls here, so that we'll get asked about the
  * pincode instead of being told the pincode */
 g_dbus_method_invocation_return_dbus_error (invocation,
          "org.bluez.Error.Rejected",
          "Rejected bluetoothd generated PIN code");
}