value property is doc'ed to always be synched to the selectedItem of the selection model. That synch is broken when setting a SelectionModel that has a selectedItem. Either the box itself or its skin (or maybe both) is broken in not updating the value.

    /**
     * Standalone test: setting a selectionModel which has a selected item
     * must update the value
     */
    @Test
    public void testSetSelectionModelUpdatesValueStandalone() {
        ObservableList<String> items = FXCollections.observableArrayList(
                "9-item", "8-item", "7-item", "6-item",
                "5-item", "4-item", "3-item", "2-item", "1-item");

        ChoiceBox box = new ChoiceBox(items);
        SingleSelectionModel model = new SingleSelectionModel() {
            @Override
            protected Object getModelItem(int index) {
                if (index < 0 || index >= getItemCount()) return null;
                return box.getItems().get(index);
            }

            @Override
            protected int getItemCount() {
                return box.getItems() != null ? box.getItems().size() : 0;
            }
            
        };
        // just to be on the safe side in case the skin/behaviour is
        // responsible for the update
        // doesn't make a difference, though
        StageLoader loader = new StageLoader(box);
        int index = 2;
        model.select(index);
        assertEquals("sanity: model is selecting index and item", items.get(index),
                model.getSelectedItem());
        box.setSelectionModel(model);
        assertEquals("box value must be same as selected item", items.get(index), box.getValue());
    }