Run the code:

    PieChart testedPieChart = new PieChart();
    VBox vb = new VBox();

    @Override
    public void start(Stage stage) throws Exception {

        ObservableList<PieChart.Data> data = FXCollections.<PieChart.Data>observableArrayList();
        for (int i = 0; i < 4; i++) {
            data.add(new PieChart.Data("Data item " + i, i * 100));
        }
        testedPieChart.setData(data);

        testedPieChart.setTitle("PieChart");
        testedPieChart.setStyle("-fx-border-color: darkgray;");
        testedPieChart.setPrefSize(600, 600);

        Pane pane = new Pane();
        pane.getChildren().add(testedPieChart);

        vb.getChildren().addAll(pane);

        Button printData = new Button("Print");
        printData.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent t) {
                List<String> styles = new ArrayList<>();
                recursiveSearch(testedPieChart.getChildrenUnmodifiable(), styles);
                System.out.println(styles);
            }

            private void recursiveSearch(ObservableList<Node> nodes, List<String> styles) {
                for (Node node : nodes) {
                    if (node instanceof Parent) {
                        recursiveSearch(((Parent) node).getChildrenUnmodifiable(), styles);
                    }
                    for (String str : node.getStyleClass()) {
                        if (str.contains("data")) {
                            styles.add(str);
                            System.out.println(node);
                            break;
                        }
                    }
                }
            }
        });

        Button addData = new Button("Add data");
        addData.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent t) {
                testedPieChart.getData().add(2, new Data("new data", 10));
            }
        });

        vb.getChildren().addAll(printData, addData);

        pane.setPrefSize(600, 600);

        Scene scene = new Scene(vb, 700, 700);
        stage.setScene(scene);
        stage.show();
    }

Click "add data" and "print".

output:
Region@68d5c08f[styleClass=chart-pie data0 default-color0]
Region@460847e7[styleClass=chart-pie data1 default-color1]
Region@6266458b[styleClass=chart-pie data2 default-color2]
Region@2d2c6ed8[styleClass=chart-pie data3 default-color3]
Region@22e08e84[styleClass=chart-pie data2 default-color4]
Region@5015bc46[styleClass=chart-legend-item-symbol chart-pie data0 default-color0 pie-legend-symbol]
Region@1e2adaa6[styleClass=chart-legend-item-symbol chart-pie data1 default-color1 pie-legend-symbol]
Region@469d4b24[styleClass=chart-legend-item-symbol chart-pie data2 default-color4 pie-legend-symbol]
Region@367bbdc6[styleClass=chart-legend-item-symbol chart-pie data2 default-color2 pie-legend-symbol]
Region@5e2b2b16[styleClass=chart-legend-item-symbol chart-pie data3 default-color3 pie-legend-symbol]
[data0, data1, data2, data3, data2, data0, data1, data2, data2, data3]

What we see: there are 2 items "data2" in each, so it becomes not unique. I expect this style class item to be unique.