The issue bubbled up at SO https://stackoverflow.com/q/70134511/203657

To reproduce, run the example, drag the first tab to the end and note the output:

expected:
0 -> 2
1 -> 0
2 -> 1

actual:
0 -> 0
1 -> 1
2 -> 2

The example:

/**
 * https://stackoverflow.com/q/70134511/203657
 */
public class TabPaneNotificationIssue extends Application {

    private Parent createContent() {
        TabPane tabPane = new TabPane(new Tab("Tab A"), new Tab("Tab B"), new Tab("Tab C"));
        tabPane.setTabDragPolicy(TabDragPolicy.REORDER);
        tabPane.getTabs().addListener((ListChangeListener<Tab>)change -> {
            while (change.next()) {
                if (change.wasPermutated()) {
                    for (int oldIndex = change.getFrom(); oldIndex < change.getTo(); oldIndex++) {
                        System.out.println(oldIndex + " -> " + change.getPermutation(oldIndex));
                    }
                }
            }
        });
        BorderPane content = new BorderPane(tabPane);
        return content;
    }

    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(createContent()));
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }

}

As a unit test:

    @Test
    public void testReorderNotificationIndexStandalone() {
        TabPane tabPane = new TabPane();
        tabPane.getTabs().addAll(new Tab("A"), new Tab("B"));
        List<Tab> copy = List.copyOf(tabPane.getTabs());
        TabObservableList<Tab> tabs = (TabObservableList<Tab>) tabPane.getTabs();
        List<Change> changes = new ArrayList<>();
        tabs.addListener((ListChangeListener) changes::add);
        tabs.reorder(copy.get(0), copy.get(1));
        assertEquals(1, changes.size());
        Change c = changes.get(0);
        assertTrue(c.next());
        assertTrue("change must be permutation", c.wasPermutated());
        assertEquals("from", 0, c.getFrom());
        assertEquals("to", 2, c.getTo());
        for (int oldIndex = c.getFrom(); oldIndex < c.getTo(); ++oldIndex) {
            int newIndex = c.getPermutation(oldIndex);
            Tab tab = copy.get(oldIndex);
            assertEquals("permutation must return index of tab " + tab.getText() + " in permutated list",
                    c.getList().indexOf(tab), newIndex);
        }
    }

BTW: not sure if it's okay to assign this to you Ambarish, but I think you have the most experience with TabPane :)