To reproduce:

1. Select Root node.
2. Push Ctrl + Shift + PgDown.
All visible items are selected.
3. Push Ctrl + Shift + PgDown.
Nothing happens. It must have selected the rest of items.
4. Push Ctrl + Shift + PgDown.
Only now it selects the rest 0f items.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.MultipleSelectionModel;
import javafx.scene.control.SelectionMode;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;

public class TreeViewCtrlShiftPgDown extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        Pane root = new Pane();

        TreeView<String> treeVeiw = new TreeView<>();
        treeVeiw.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE);
        treeVeiw.setPrefWidth(300d);
        treeVeiw.setPrefHeight(300d);
        
        TreeItem rootItem = new TreeItem("Root");
        rootItem.setExpanded(true);
        treeVeiw.setRoot(rootItem);

        for (int i = 0; i < 4; i++) {
            TreeItem treeItem = new TreeItem("item-" + i);
            treeItem.setExpanded(true);
            rootItem.getChildren().add(treeItem);
            for (int j = 0; j < 4; j++) {
                TreeItem childTreeItem = new TreeItem("item-" + i + "-" + j);
                treeItem.getChildren().add(childTreeItem);
            }
        }

        root.getChildren().add(treeVeiw);
        
        Scene scene = new Scene(root, 400, 300);
        stage.setScene(scene);
        stage.show();
    }
}