See example below that demonstrates the issue which has the same reason as RT-15793 and is marked as temporarily fixed for comboBox as well.

    /**
     * - sanity: open popup and see it's empty (expected)
     * - press button
     * - open popup
     * - expected: see added item
     * - actual: empty
     */
    public class Combo15793Regression extends Application {
    
        @Override
        public void start(Stage primaryStage) throws Exception {
            ComboBox box = new ComboBox();
            Button button = new Button("set empty list and add item");
            button.setOnAction(e -> {
                box.setItems(FXCollections.observableArrayList());
                box.getItems().add("Toto");
            });
            Parent root = new VBox(box, button);
            primaryStage.setScene(new Scene(root));
            primaryStage.setTitle(System.getProperty("java.version"));
            primaryStage.show();
        }
    
        public static void main(String[] args) {
            launch(args);
        }
    }

Didn't try earlier versions to verify if it really worked for ComBox (the bug was reported against ListView), though the code looks like it should have:

    // FIXME temporary fix for RT-15793. This will need to be
    // properly fixed when time permits
    if (getSkin() instanceof ComboBoxListViewSkin) {
          ComboBoxListViewSkin<?> skin = (ComboBoxListViewSkin<?>) getSkin();
//----> technically missing: skin.updateComboBoxItems();
// which might have been added later than this hack
          skin.updateListViewItems();
     }

culprit is the missing update of skin's alias to comboBoxItems

    public void updateListViewItems() {
       ...
        // sets list items to old comboBox items
        this.listViewItems = comboBoxItems;
        listView.setItems(listViewItems);
       ....
    }