title says it all :) Once a skin has been attached to the control, replacing the control's selectionModel keeps a reference to the old selectionModel's selectedItem.

the offending lines in TabPaneSkin constructor (commenting them will let the test pass):
        registerChangeListener(control.getSelectionModel().selectedItemProperty(), e -> {
            isSelectingTab = true;
            selectedTab = getSkinnable().getSelectionModel().getSelectedItem();
            getSkinnable().requestLayout();
        });


Failing test case (in SelectionFocusModelTest that's added in the fix for JDK-8241455, this method effectively ignored until this is fixed)

    @Test
    public void testTabPaneSelectionModel() {
        TabPane control = new TabPane();
        ObservableList<String> data = FXCollections.observableArrayList("Apple", "Orange", "Banana");
        data.forEach(text -> control.getTabs().add(new Tab("text")));
        WeakReference<SelectionModel<?>> weakRef = new WeakReference<>(control.getSelectionModel());
        SingleSelectionModel<Tab> replacingSm = TabPaneShim.getTabPaneSelectionModel(control);
        showControl(control);
        control.setSelectionModel(replacingSm);
        attemptGC(weakRef, 10);
        assertNull("selectionModel must be gc'ed", weakRef.get());
    }